A coalition of 97 child health advocates sent a letter to Mark Zuckerberg on Tuesday asking him to discontinue Messenger Kids, a new advertising-free Facebook app targeted at 6-to-12-year olds. Advocates say the app likely will undermine healthy childhood development for preschool and elementary-school-aged kids by increasing the amount of time they spend with digital devices.

The letter to Zuckerberg was signed by individuals and 19 nonprofits including Common Sense Media, Campaign for a Commercial-Free Childhood, and Parents Across America, who say their concern stems from recent studies that link increased depression, poor sleeping habits, and unhealthy body image in children and teens with higher use of social media and digital devices.

For instance, a study by Jean Twenge, a psychology professor at San Diego State University and author of iGen, found that social media use by teens is tied to significantly higher rates of depression. (Twenge signed the letter.) Another recent study found that adolescents who spend an hour a day chatting on social media report less satisfaction with nearly every aspect of their lives and 8th graders who use social media for six to nine hours per week are 47 percent more likely to report they are unhappy than their peers who use social media less often.

“Raising children in our new digital age is difficult enough,” the letter says. “We ask that you do not use Facebook’s enormous reach and influence to make it even harder.”

Facebook has said that it took precautions with Messenger Kids, including barring advertising, and giving parents more tools to control a child’s social media use. “But even if these safeguards are effective,” the letter says, “the app’s overall impact on families and society is likely to be negative, normalizing social media use among young children and creating peer pressure for kids to sign up for their first account.”

The letter adds to growing concerns about the impact of social media and smartphones on our minds and bodies. In January, two major Apple shareholders wrote a public letter to the company, citing some of the same studies, and asking Apple to address potential negative mental and physical effects of smartphone usage on children, including funding research and building better tools for parents.

The message to Zuckerberg, however, strikes a much less conciliatory tone. Advocates poked holes in Facebook’s stated motivation for launching the app, and pointed out features that were seemed to be designed for Facebook’s benefit.

When Facebook launched Messenger Kids in December, the company pitched it as a way to safeguard pre-teens who may be using unauthorized social media accounts. (The Children’s Online Privacy Protection Act, or COPPA requires parental permission to collect data on children under 13.) But advocates say pre-teens who already have a Snapchat, Instagram, or Facebook account are unlikely to convert, especially when features are designed with younger users in mind. Instead, they say the app seems designed to hook children on social media at younger ages.

Josh Golin, executive director of Campaign for a Commercial Free Childhood, the nonprofit that helped organize the coalition, pointed to comments from David Marcus, Facebook’s head of Messenger. When the kids version launched, Marcus told TechCrunch that Facebook hired a special team to build creative tools for kids, like fidget spinner and dinosaur augmented reality masks, as well as crayon-style stickers. He said the filters would allow children to conduct longer conversations with grandparents, for example.

But iIf a 7-year-old can’t chat longer than 5 to 10 minutes, why extend it, says Golin. “Using filters and AR in order to extend the chat will only make it harder for kids to have real conversations without gimmicks in both the short and long run,” he says. “So here’s Facebook framing increased use and dependence on its tools as a benefit to 7-year-olds and grandparents when really its Facebook that’s the beneficiary.”