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

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"

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.  *       

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