Brands are also pursuing children with smaller followings . The toy company Melissa & Doug emailed parents about a six-week influencer campaign last summer, offering payments and free toys for weekly Instagram posts of their children “having fun with the toys!” The company said it would pay $10 per 1,000 followers for individual Instagram posts and $5 per 1,000 followers for Instagram Story posts.

The rise of this kind of advertising has raised questions involving fair compensation, oversight and work permits, especially since child labor guidelines vary by state.

Andrea Faville, a YouTube spokeswoman, said that the site didn’t allow anyone under 13 to make or own accounts and that it worked “closely with experts, nonprofit organizations and others in our industry to protect families using our services.”

YouTube came under fire last month after lewd comments by pedophiles were discovered on innocent videos of children; the company has since said it will suspend comments on most videos of minors. Some channels that can “demonstrate a low risk of predatory behavior” will keep comments but require moderators, the company said. It remains to be seen if disabling comments will hurt the kind of connections kidfluencers try to establish with their fans.

Sravanthi Dev, a spokeswoman for Instagram, said that while the platform prohibited users 12 and under, their parents or representatives could create profiles for them “as long as it is clear in the bio information that the account is run by the parent or representative.”

Michelle Foley’s 6-year-old daughter, Ava, and her best friend, Everleigh , also 6, have more than a million followers on their shared Instagram and YouTube accounts. YouTube’s analytics say Ava and Everleigh’s viewers are largely between the ages of 25 and 44, Ms. Foley said, but she said she thought the core audience was between 8 and 18.