How to prevent kids from locking apps with Face ID?

My son, who is 12, is locking and hiding apps with face ID, saying that kids at school are grabbing each other’s phones (in the 15 mins before they get locked for the day by the school) and sending silly messages from them. Whilst he readily agreed to remove them, I’m concerned that he is able to do it at all given the Apple policy on this says:

“Note: Children under 13 in a Family Sharing group can’t lock or hide apps.”

He is set up as a child with his correct age so not sure what has happened. Is there a way to prevent this in screen time settings? I have already disabled changes to Face ID but this seems to have no impact on the individual app locking…