How do I add my child’s number to family sharing after it’s set up
How do I add my child’s phone number to family sharing after I’ve already set it to my number?
You add their number to their Apple ID. You need to 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.