How do I change a family member from adult to child?

If you mean change their date of birth so that they are under-13 (only under-13 accounts are classified as child accounts) then log into the account on http://appleid.apple.com and change it : Update the birth date associated with your Apple ID - Apple Support

To change your birth date to any age below 13*, you must be a member of a Family Sharing group, and your family organizer must approve the change. After updating your birth date, follow the onscreen steps to send an email request to your family organizer. Your family organizer can sign in to their Apple ID account page to confirm the birth date change and agree to the Parent Privacy Disclosure agreement by entering the requested information from their account payment method.

Be sure to enter your correct date of birth. After you change it to any age below 13, you can't edit it.