The app, launched in 2017, is aimed at under-13s and is meant to be used as a safe space for them to chat to friends and family, controlled from a parent’s main Facebook account.
It gave parents the ability to approve the people their children could chat with, but a design flaw has been found which made it possible for unauthorised users to be part of group chats.
Facebook said the flaw – which has now been fixed – meant that in some cases it was possible for a child in a group chat to come into contact with a third person who, while approved by the parent of one child, had not been approved by the other.
“We recently notified some parents of Messenger Kids account users about a technical error that we detected affecting a small number of group chats,” a Facebook spokesman said.
“We turned off the affected chats and provided parents with additional resources on Messenger Kids and online safety.”
Messenger Kids is not available in the UK but is open to users in the US, Canada, Peru and Mexico.
At the time of the app’s release in December 2017, Foreign Secretary Jeremy Hunt – then Health Secretary – accused Facebook of failing to act responsibly when it came to protecting children.
“Not sure this is the right direction at all,” Mr Hunt tweeted at the time.
“Facebook told me they would come back with ideas to PREVENT underage use of their product, but instead they are actively targeting younger children. Stay away from my kids please Facebook and act responsibly.”
The social network said at the time of the launch that the app had been created after consultations with parents and safety experts and had been created in response to parents “increasingly allowing their children to use tablets and smartphones”.