How to add a family member to apple subscription
How to add a family member to Apple family sharing
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This should help.
Set up Family Sharing - Apple Support
Children are a special sub-set within Apple Family.
An AppleID for a child can NOT be “created out of thin air” as they can for an adult.
A child MUST be created by a family organizer AND must always be in a family. (No orphans are allowed)
Create an Apple ID for your child - Apple Support
Lastly, DO be VERY careful about setting a child’s birth date accurately; an incorrect date WILL MATTER as the child gets older and it is VERY difficult to change.