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.”