Removing a child family member
How do you remove a child from the family group?
Accounts with an age set to under 13 years old, cannot be removed from a Family Group, only moved to another Family Group. That is accounts under 13 cannot exist outside of a Family Group.
If you need to completely delete a child account then you need to contact Apple Support. click here-> Get Support
For accounts with an age over 13, then you can follow the steps here: link-> Remove someone from Family Sharing - Apple Support
Do note, this does not delete the account. The removed account will still exist on its own and can be used.
If there is an error in the account, its better to fix it, than to remove it from Family Sharing.