How do I remove a child from family sharing under the age of 13

You can't remove a child under 13 from Family Sharing. You can either:

Move a child to another group using Family Sharing - https://support.apple.com/HT201080

or:

Delete their Apple ID account entirely.

One way to do this would be to log into the unwanted "child" account and request deletion of the AppleID. 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.

Family Sharing and Apple ID for your child - https://support.apple.com/HT201084

- "Your email address is the rescue email address for your child’s account, and you can use it with the security questions you provided to reset a forgotten password. You can keep your your child's account more secure with two-factor authentication."

- "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 privacy.apple.com."

How to delete your Apple ID account - https://support.apple.com/HT208504

- Can I delete my child’s account?

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

You could also contact Apple about this request: Click here --> https://support.apple.com/choose-country-region/media-services Select your country, then a product and work your way down the menu to the desired issue. If you don't see one then try a different product until you reach a telephone or chat help request and ask to be redirected.