The app, launched in 2017, is aimed at under-13s and is meant to be 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 was found which made it possible for unauthorised users to be part of group chats.
In some cases a child in a group chat could come into contact with a third person who, while approved by the parent of one child, had not been approved by the other.
The flaw in Messenger Kids – an app not available in the UK but open to users in the US, Canada, Peru and Mexico – has since been fixed.
In a letter signed by senators Edward Markey and Richard Blumenthal, the social network was asked whether it has done enough to protect children, and for clarity on when it first became aware of the issue.
“We write seeking more transparency regarding the nature of the Messenger Kids flaw and answers on what Facebook is doing to ensure that the product meets the expectations of parents and its obligation under law,” the pair wrote.
“Children’s privacy and safety online should be Messenger Kids’ top priority.
“Your company has a responsibility to meet its promise to parents that children are not exposed to unapproved contacts, a promise that appears that Facebook has not fulfilled.”
The letter asked the social network to answer nine specific questions by August 27.
When the flaw came to light at the end of July, Facebook said: “We recently notified some parents of Messenger Kids account users about a technical error that we detected affecting a small number of group chats.
“We turned off the affected chats and provided parents with additional resources on Messenger Kids and online safety.”