After it launched with mixed reviews, more than 100 child development experts, health advocacy groups, educators, and parents have called on Facebook to shut down Messenger Kids, a spinoff of the company’s messenger app that’s specifically designed for children ages six to 12. The collective, led by the Campaign for a Commercial-Free Childhood, addressed its open letter to CEO Mark Zuckerberg, warning that the app is “harmful to children and teens,” and that it could “undermine children’s healthy development.”

The app, which debuted in December, was created as a workaround for parents who want to communicate with their children via Facebook, but cannot use the regular version since federal law prohibits children under the age of 13 from signing up for the service. Messenger Kids requires parents to create the accounts and allows them to control exactly who their kids can contact through the app. On it, kids can use Facebook’s video and GIF tools to send child-friendly media to their family members, or make calls to the approved contacts. The app also contains no ads, in-app purchases, and does not require the child to use their real name — unlike the original version of Facebook.

But despite these data-specific safeguards, the group says “the app’s overall impact on families and society is likely to be negative,” as it creates peer pressure for kids to join a digital social network.

“Younger children are simply not ready to have social media accounts,” the letter reads. “They also do not have a fully developed understanding of privacy, including what’s appropriate to share with others and who has access to their conversations, pictures, and videos. At a time when there is mounting concern about how social media use affects adolescents’ well-being, it is particularly irresponsible to encourage children as young as preschoolers to start using a Facebook product.”

Facebook’s global head of safety, Antigone Davis, responded to the letter by saying it collaborated with child health experts to make Messenger Kids as safe for kids as possible. “We worked to create Messenger Kids with an advisory committee of parenting and developmental experts, as well as with families themselves and in partnership with National PTA,” she told The Washington Post. (Facebook’s original blog post announcing the product contains a disclaimer saying that the National PTA does not actually endorse any of Facebook’s commercial products.)

Today’s open letter adds to a series of growing concerns over the impact of social media, particularly on younger users. Before Messenger Kids’ launch, former Facebook president Sean Parker criticized the social network’s intentionally addictive design. “God only knows what it’s doing to our children’s brains,” he said last November. “It’s exactly the kind of thing that a hacker like myself would come up with, because you’re exploiting a vulnerability in human psychology.”

Analytics company Apptopia estimates that Messenger Kids has roughly 45,000 downloads since its launch, with about 20,000 daily users.