Facebook boss Mark Zuckerberg has been petitioned in an open letter, which has accused the social media firm's Messenger Kids service of being an "irresponsible" way of encouraging young children to use its website.
The open letter reads: "Messenger Kids will likely be the first social media platform widely used by elementary school children [four- to -11-year-olds].
"But 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.
"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."
Facebook, for its part, has pointed out that the app was designed with online safety experts.
The firm said: "Since we launched in December we've heard from parents around the country that Messenger Kids has helped them stay in touch with their children and has enabled their children to stay in touch with family members near and far.
"For example, we've heard stories of parents working night shifts being able read bedtime stories to their children, and mums who travel for work getting daily updates from their kids while they're away."