For more details, see the FAQ at https://messengerkids.com/
"Analysts say that messaging apps give Facebook a chance to tap into a younger generation that they have been losing; earlier this year, the company bought the popular teen messaging app tbh. When Facebook asked parents about launching a kids' app, many told the company that they did not want a full social network, but had more interest in a communications tool, the company said.Facebook’s new messaging app deepens debate over kids’ social-media use - The Washington Post
Facebook said that Messenger Kids will have no ads. It will also not use data from Messenger Kids for Facebook ads. (Parents shouldn't, for example, see an ad for a toy on Facebook because their child talked about it on Messenger Kids.) The firm said no data from Messenger Kids will be fed to the main social network, nor will their information automatically port to other Facebook products when they turn 13, the company said."