You are welcome. You can't remove a child under 13 from Family Sharing. You might try contacting Apple ID Support to see if they can/will delete the unwanted ID. From the article below.
Your child must remain part of your family group until they turn 13.* However, if necessary, you can transfer a child to a different family group.
Family Sharing and Apple ID for your child - Apple Support
Apple Support by e-mail or chat Select your device, Battery, Power and Charging, then Power adapter not working to get a chat option. If you need to talk to them, ask them if they will do a Skype or FaceTime with you.