How to remove a child's account from Family Sharing?

The child either has to be transferred to a different sharing group or their Apple Account has to be deleted entirely with loss of everything associated with it.

See this article about how to move a child to another group using Family Sharing - Move a child to another group using Family Sharing - Apple Support

I presume this is a minor age child? The document Move a child to another group using Family Sharing - Apple Support says: "You can't remove a child under 13 years old [see the document for regional age variations] from your family group, but you can move them to another group or delete their account." If this is a non-minor child you can remove them from your group as outlined in this document Leave Family Sharing - Apple Support

The alternative is deleting the child's account altogether which has the consequence that anything purchased with that account will not be available for redownloading or updating in the future. One way to delete the account would be to log into the unwanted "child" account and request deletion. You should have the ID and password from when you first set up the account, or can recover the password according to information provided in the referenced links below:

Document: Create an Apple ID for your child - Create an Apple Account for your child - Apple Support

- The section: Reset a child's Apple ID password

- If you need to see the options you have for managing your child's account, you can sign in with their Apple ID and password at https://privacy.apple.com/account."

Document: How to delete your Apple ID account - How to delete your Apple Account - Apple Support

- Can I delete my child’s account?

"Your child can also request deletion of their Apple ID, which requires your approval."