To participate in Family Sharing, all family members must have their own Apple ID. Children under 13* can't create an Apple ID on their own. However, as a parent or legal guardian, the family organizer can provide verified parental consent for a child to have their own Apple ID, then create it on the child’s behalf.
When you create an Apple ID for a child, it will be added to your family group automatically.
One of the nice things that Apple has done is made it possible for kids to transfer their account to another group once they are 13.