How can I remove a child from my Apple Family Sharing plan?

My ex and his son used to all be a part of my apple family share plan. As his son is under 13, I am unable to simply remove him from my family share plan. I have asked my ex repeatedly to remove him and he refuses, simply because he wants to be difficult.

The password for the account is set up and linked to my ex's phone number, so I can't delete it. IS THERE ANY OTHER WAY TO REMOVE THIS CHILD FROM MY ACCOUNT???

[Re-Titled by Moderator]