Mila and Emma Stauffer, 2-year-old twin sisters who live with their parents and three older siblings near Phoenix, were sitting on the floor, having a discussion about what they might be when they grow up. “Maybe a teacher?” Emma said.

“Emma, you hate kids!” Mila said.

“How about a doctor,” Emma suggested.

“Emma, you hate blood,” her sister said.

“Oy yep, I hate blood,” Emma said.

The video of this conversation was filmed by their 14-year-old sister, Kaitlin, this past summer. It appeared originally on their mother’s Instagram page, where it has since been viewed 4.4 million times.

Interactive Feature | socialkids1

Mila and Emma are two breakthrough stars of a new class of social media celebrities: young children who appear in viral videos. In many of the most popular clips, these whippersnappers engage in adultlike conversations, amusingly given their babyish voices. The videos can be incredibly popular. And marketers have noticed.

Mila and Emma have done advertising work for Amazon, Nest, Dollar Rental Car, Macy’s and Walmart, among other companies. They’re flying to New York in October to shoot video spoofs of movies including “Clueless” and “Mean Girls” for Harper’s Bazaar.

“It is really lucrative,” said Katie Stauffer, their mother. “But I wish people knew that this is my job now.”

She wouldn’t detail exactly how much money the children are bringing in, but she said she was recently able to leave her position as an escrow officer after 12 years, much to the relief of herself and her husband, a doctor. With her working outside the home, “my kids weren’t getting what they needed,” she said.

Still, Ms. Stauffer often gets criticized in the comments of her Instagram feed, where people frequently remark on the food she lets her children eat and the brands she takes money from. After a sponsored post for a brand of chicken, “we got a lot of flak,” she said.

And the talent can be difficult. Emma doesn’t love making videos, and Mila wants to make them only when she wants. Ms. Stauffer has stopped cutting deals with companies that insist on giving her deadlines. “You can’t make 2-year-olds do anything,” she said.

Instead she stages photo shoots many times a week, during which the girls do relaxed toddler things: make princess cakes; drag dolls dressed like the twins; or sit in wagons while having staring contests in front of the family’s stately red-doored home.

The images are then posted to Ms. Stauffer’s 2.2 million followers. Recently a Stauffer video got a coveted repost from Kris Jenner, perhaps the ultimate authority on building daughters’ brands. “#iminlove,” Ms. Jenner wrote.

Kaitlin often cues the children their more sophisticated lines. In one recent video, Mila addresses a date with her friend Sawyer that went boringly wrong when he paid more attention to his sports league than to her.

“Fantasy football?” Mila said, while raising her palm talk-to-the-hand style. “So basic.”

Ross Smith, a 25-year-old social media star (he has four million Facebook followers, 1.5 million on Instagram, and an average Snapchat post gets about one million views), has collaborated with several children.

“Kids are the new social influencer,” he said. He is not a parent himself, but he understands the instinct to seize on corporate offers when they arise. “Kids grow up and become less relevant. The sweet spot is between 2 and 4,” after which, Mr. Smith said, “they’re not that cute.”

Mr. Smith lives in Columbus, Ohio, and is best known for videos he shoots with his 91-year-old “Granny” (whom he prefers not to name). They have worked with Mila, making a video of Granny giving her dating advice. It posted in September and has been viewed more than 31 million times on Mr. Smith’s Facebook page.

Grandma gives relationship advice to a 2 year old
Video by Ross Smith

He has also teamed up with Korbin Jackson, a 3-year-old from Dothan, Ala., who is best known for his soccer and Ping-Pong ball trick shot videos.

Their video pitted Korbin against his sparring partner (whose T-shirt said, “Straight Outta the Nursing Home”) in a trick shot battle. It has been viewed 18 million times on Mr. Smith’s Facebook page. To film it, Mr. Smith flew Korbin and his parents to Ohio. Upon arriving to Mr. Smith’s house, Korbin said he needed a nap, so the production was halted for an hour or so.

“It’s hard to work with kids, but it’s fun,” Mr. Smith said.

Korbin got his start in social media accounts after his parents made accounts to share videos and pictures of him with friends and family. His father, a former professional arena football player with his own large social media following, began to post videos of the bespectacled, ebullient child doing sports tricks.

Ten of Korbin’s videos have been featured on ESPN, and he has been interviewed by news stations in Japan and Romania. His Instagram feed has nearly 65,000 followers.

Born with a disability, Korbin spent two and a half years doing physical therapy. “He finds so much joy and happiness in life, and we are so proud to share his good energy,” said his mother, Stephanie Jackson.

She said when Korbin arrives at school with his father, the older children surround him and ask to take selfies with the local celebrity. “When Korbin gets to high school, his following will be huge,” she said. “That Instagram just set him up for greatness.”

Korbin receives a lot of free stuff in the mail, and his parents often tag the brands in posts. Unsolicited, Ms. Jackson said, GoPro sent about $2,000 worth of camera equipment. Many sportswear companies send packages. Korbin’s feed frequently promotes Under Armour. “They send him boxes once a month,” his mother said. “It’s not a paper deal, it’s not a contract, they are inspired by him.”

Aware of the existence of online predators, she tries to limit the number of people who know the family’s home address. “We do get weird emails and DMs like ‘We will meet him no matter what you say,’” Ms. Jackson said.

In May, a video of him putting out the candle on a birthday cake by kicking a ball that hits and extinguishes the flame went viral. His parents sold the licensing rights to Jukin Media, and took him to the bank to open an account in his name, where any payment related to his videos would be deposited. He has earned about $5,000. “That’s Korbin’s money, not ours,” she said.

Interactive Feature | socialkids2

Laws that regulate children’s rights to money earned by parents using their images on social media aren’t clear, lawyers say. In California, the so-called Coogan Law — named for the child star Jackie Coogan, who worked with Charlie Chaplin in “The Kid” — mandates that a portion of money earned by child actors be placed in trust for the children.

But the law is written about children being employed or placed under contract with third parties. When parents are paid by brands to post images and videos of their children on social media, or they make money from YouTube ads, are children owed anything? “These are uncharted waters,” said Anthony Amendola, a partner at the Los Angeles law firm Mitchell Silberberg & Knupp.

There are plenty of parents eager to jump in. Destiny Bennett and her husband, Devonte, moved to Los Angeles earlier this year and decided that acting would be a good career for their son Caidyn, 4.

But when he didn’t get the audition callbacks that his parents had hoped for, they decided to try marketing his talents through videos posted to Instagram and YouTube. “We told him, ‘We can do our own auditions, and you can talk about anything you want.’”

Earlier this month, they posted a video in which Caidyn laments the way people touch his dreadlocks without his permission. “I hate when I am with my mommy and people say, ‘Oh, are those braids?’ And it’s like, ‘No, Susan, those are dreads.’” Caidyn says. “But my mommy says I have to be polite.”

NO Susan! You dont touch a black mans hair
Video by Caidyn Bennett

The video was a success, giving rise to a hashtag #NoSusan and bringing a total of 15,000 followers to his personal Instagram feed. His parents have been contacted about brand sponsorships.

Ms. Bennett says Caidyn is not yet sure if he will continue to focus on making viral videos, or the clothing line that she says he is preparing with his brother, who is 1.

“We are a family of entrepreneurs,” she said.

Josh Gaines, known on social media as “Josh Darnit,” previously used the six-second video loop platform Vine, which was introduced on Twitter in 2013, to post videos of his children, most successfully his young son Evan smacking his dad. Making Vine videos was easy, he said, and he could repackage them on YouTube to generate revenue in addition to sponsorships.

When Vine shut down in 2016, Mr. Gaines recalibrated his focus on videos for Facebook, YouTube and Instagram. These videos require more production resources. Evan, now 7, has grown bored. “What I don’t like about doing them is they take a superlong time, and sometimes Dad tells me to say hard words. Like ‘subscribe,’” Evan said.

Fortunately, his older sister, Johnna, 11, has stepped up. “Once we got to that age that Evan resisted a lot, I shifted more to my daughter,” Mr. Gaines said. “She is a little workhorse.”

Improvised videos are easier. “I like to be in the spotlight,” Johnna said. “If we have to do a script and Dad is telling us what to say, like an ad for a toy, or when we were doing the ones for the drones that had a script, that felt more like work.”

For a deal he cut with Best Buy, Mr. Gaines enticed Evan and Johnna to spend their last week of summer vacation taking a road trip to a Nintendo World Championship qualifiers, which were taking place at a few Best Buy stores.

Interactive Feature | Of the Moment The lifestyle newsletter from the Styles, Travel and Food sections, offering the latest trends to news you can use.

On the trip, Mr. Gaines would post pictures and videos to Twitter, Instagram and YouTube. Each of the children got a new Nintendo Switch game system, he said.

He and his wife, Sarah, said they don’t pay the children or split revenue with them. The family’s standard of living is improved when the social media business is going well, and all members benefit, they said.

Best Buy wanted to team with the “Darnit Family” because they have broad reach across many digital platforms and are known for family fun. “They are on a road trip as a family and are having fun through Best Buy, and that’s exactly the message we want people to get excited about,” said Shane Kitzman, a Best Buy spokesman. He would not disclose any financial details about the partnership.

Sponsorships and monetization were not on Katie Ryan’s mind when she started capturing snippets of her daughter Ava and posting them on Vine. Ms. Ryan’s sister died and her sharing clips that showed her daughter’s emerging sense of humor was a balm for the whole family.

Ms. Ryan didn’t realize that Ava’s proclivity to create characters and do deadpan impersonations of them would attract a wider audience.

In the videos, Ava sits in the living room, playing Charlene, “the hot mess,” who says things like, “It’s O.K. for you to have a wedding but when I want to have a party to show my love for nachos, nobody cares.” Or Ava is at a computer at the kitchen table, where Bossy Boss Lady, based on someone Ava noticed at her great-aunt’s office, says: “Look at this great view I have. Of Carol eating Cheez-Its. All day”.

Interactive Feature | socialkids3

The videos consistently go viral. The Instagram account (it is in her mother’s name) has 717,000 followers.

Thanks to the ad revenue she earns via YouTube, Ms. Ryan said she is able to be a stay-at-home mother.

She looks for exciting experiences for Ava, who is 7. In August, W magazine flew Ava and her mother to New York to shoot a video of Ava taking on the character of a fashion editor, uttering phrases including “This is what I wear when I eat a big pint of ice cream. Wait, I haven’t eaten ice cream in 17 years.”

The video has been viewed 35 million times across various Condé Nast magazine Facebook pages, according to Anne Sachs, W’s executive digital director.

Though Ava writes her ideas for video in a journal and will ask to do them regularly, “sometimes I tell her, ‘Let’s just got to the park instead of doing a video right now,’” Ms. Ryan said. “This is like her little side project in life. She’s happy and that’s all that matters.”

Zoie Fenty, a former Starbucks shift supervisor in Atlanta who now earns a living making videos, also finds joy in children’s online performances.

Mr. Fenty has done mash-ups of him “FaceTiming” with youngsters in viral videos, including Mila. As she described her experience of visiting a preschool (“The kids are insane, throwing staplers, pooping everywhere!”) Mr. Fenty appears in the bottom left corner of the screen, shaking his head in sympathy and horror. (“What? Ohmaga!”)

“Kids are entertaining, they don’t care what anyone thinks,” Mr. Fenty said in a phone interview. “There used to be a show called ‘Kids Say the Darndest Things.’ This is it today.”