"A growing body of research demonstrates that excessive use of digital devices and social media is harmful to children and teens, making it very likely this new app will undermine children's healthy development," they wrote.
"Younger children are simply not ready to have social media accounts. They are not old enough to navigate the complexities of online relationships, which often lead to misunderstandings and conflicts even among more mature users. 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."
Others are more positive about the app, arguing it provides a safe space for children to learn how social media works. TechCrunch's Sarah Perez wrote: "The alternative to using Messenger Kids is what a lot parents do - they refuse all social apps until kids reach a certain age, then throw them to the wolves on the internet. Is that really better?"
In Friday's announcement, Facebook said: "We designed Messenger Kids from the ground up with elements that teach kids how to better understand and express their emotions in creative ways, encourage and promote healthy social behaviors, and deepen positive connections between kids and their close friends and families."
It has announced a number of new features for the app. There's now a "Pledge" that it asks parents to read and agree to with their children, including "be respectful" and "be safe." It's also adding "Kindness stickers" that are "designed to inspire kindness towards others," and Appreciation Mission, "an interactive guide within the app that will encourage kids to discover and express appreciation for their friends and family."
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"We are committed to building better products for families, including Messenger Kids. That means listening to parents, experts, and our critics. It's why we engaged in conversations with many groups in the year leading up to the launch and we have continued doing so this year as we prepared to bring the app to families in Canada and Peru," a Facebook spokesperson tells Business Insider.