Apple ID

See Family Sharing and Apple ID for your child under 13 years https://support.apple.com/en-us/HT201084

And see See How to create a new Apple ID https://support.apple.com/en-us/HT204316

If 13 years old or older.

Apple ID - create a new.      

Apple ID – Create         

If under 13 years old, you will need to set up Family Sharing to create an ID.

Family Sharing and Apple ID for your child.      

Family Sharing.         

Family Sharing - Set up.