My child already has an apple id, how can I add him to family sharing

Same problem! When I add them as a child it says the is already in use and to choose another ID?!

Try adding the existing ID as an adult. After they are added, change their birthdate their correct age (below 13 to make their ID a child's ID). Link to contact Apple Support at the end of the article.

Apple ID - Update the birth date associated with.  *       

Click on this link to learn how to 'Set up Family Sharing' --> https://support.apple.com/HT201088

Read the section: "Invite people to join your family"