The feature is strictly opt-in and should be available starting today.

The addition shows Facebook trying to walk a fine line. It wants to expand Messenger Kids’ audience so that more children switch to the regular Messenger app when they turn 13, but it also has to honor strict privacy protections for children. It won’t please critics, however. Opponents like the Campaign for a Commercial-Free Childhood have pointed to research warning that overuse of digital devices may be harmful to children, and that younger kids aren’t mature enough to deal with the nuances of online relationships. These objectors see very existence of Messenger Kids is a problem, and no amount of improvements are likely to satisfy them.

[SOURCE]