Strictly speaking, only those aged 13 and over are allowed to use Facebook. But the prevention methods are trivial, meaning more than 20 million under-13-year-olds are thought to be using the network.
So on Monday, Facebook launched its first app tailored for young users. It's a ringfenced network that needs parental approval before use, and will not - the company has promised - be used to feed data for advertising.
Messenger Kids is a simplified, locked-down version of the messaging app Facebook today offers those over 13.
"Parents are increasingly allowing their children to use tablets and smartphones, but often have questions and concerns about how their kids use them and which apps are appropriate," said Loren Cheng, product manager for Messenger Kids.
"So when we heard about the need for better apps directly from parents during research and conversations with parents, we knew we needed to develop it alongside the people who were going to use it, as well as experts who could help guide our thinking."