Family Sharing
How do I add a child to family sharing that already has an Apple ID?
iPhone 11 Pro Max
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Use the steps in the article below. Invite that ID to join. If the child is currently part of another family and is less that 13 years old, try the 2nd link. If the child is currently part of another family and is 13 years old or older, they must leave the current family using the steps in the 3rd article before you can invite them to join.
Family Sharing - Transfer a child to a different family group - Apple Support?