How do you change an adult apple ID to a "child"?

I am the organizer of my family account. We have a 20 year old daughter, whom we have full legal guardianship of, who recently got her own phone (and apple ID). How do I change her "adult" account to a "child" account so that we can limit her interactions and time on social media? She has placed herself in many dangerous situations. Other than taking the device away, which will teach her nothing, we want monitor.