Change phone number for child account in Family Sharing
I setup my Family Sharing account for my child before they had a phone number, now they have one. How can I update it on their account?
You need to change/add the number on their Apple ID. Look in the Security section when signed in online. After the change, allow up to 24 hours for the change to sync. Before trying to make the change, sign out of Messages/FaceTime on all devices/computers.