How do I remove an under 13 years old child from Family Sharing?

You can only transfer them to another Family Account or delete their account. A child under 13 cannot have an account on a device that is not managed in a family account. Both options are available here:

Move a child to another group using Family Sharing - Apple Support

I’ll only add to read and follow the instructions VERY carefully.

Easily overlooked; “Invite in Person” and that the child’s account access credentials must be be entered into the “gaining” organizer’s device before the electronic invite is sent to the “loosing” organizer.