The company said it fills "a need for a messaging app that lets kids connect with people they love but also has the level of control parents want."
But a group of 100 experts, children's advocates and parenting organizations disagrees. "Messenger Kids is not responding to a need — it is creating one," the letter says. "It appeals primarily to children who otherwise would not have their own social media accounts." Another passage criticized Facebook for "targeting younger children with a new product."
Experts are concerned about the effects that smartphones and social media apps have on people's physical and mental health. A recent report said teenagers are happier when they spend less time on screens.
Many preteens have found their way onto Facebook, Snapchat and Instagram, despite rules that require users to be at least 13 years old.
Some companies have offered parental controls as a way of controlling unauthorized preteen use of their platforms. But Facebook's new kid-focused app, which features animations and emoji, seems to target a younger audience, said Josh Golin, executive director of the Campaign for a Commercial-Free Childhood.
"It looks like something that would appeal to a 6-year-old or 7-year-old," he said.
Facebook wouldn't reveal how popular the messaging app has been. But App Annie, a company that studies app use, said Messenger Kids has been downloaded about 80,000 times on iOS since it launched December 4. It has been in the top 40 most popular kids' apps since then.