The Family Sharing organizer should see every member of the Family Sharing plan in their Settings. Is the child still listed there? Both the organizer and the individual themselves can remove a person from the group, accidentally or on purpose.
Settings﹥[your name]﹥Set Up Family Sharing
An Apple Music user might get signed out of the service, if they access the iTunes Store for a different region. Unfortunately, that can happen easily by trying to listen to a track that is only available in a different region, e.g. from an internationally shared playlist. Trying to listen to such a track would present a popup with a message to change the region, and it would not be functional (as your subscription isn’t valid for that region).
Change the region back to the correct setting, and sign in again.