How to create Apple ID under family sharing for child older than 13?

You first create the apple ID (skip this step if the child already has an apple ID)

How to create a new Apple ID - Apple Support

And to invite that account to the family sharing:

Set up Family Sharing - Apple Support

See the steps under Invite people to join your family

Good luck!

Turn on Ask to Buy.

Family Sharing -  Ask to Buy.     

If they are using a device, use Screen Time if you want to add restrictions.