How can I manage iCloud backups under Family Sharing while creating a new Apple Account?

My kid is using an iPhone attached to my Apple id. If I create a new Apple id for him, switch to this new Apple id on his iPhone, and add this new Apple id to the family sharing of my Apple id, would the icloud backup/ purchase/ subscription made on my kid’s iPhone (which were made using my Apple id) be retained?

[Re-Titled by Moderator]