how can i remove my child from family sharing?

Here's a link with information you may need: Family Sharing and Apple ID for your child - Apple Support - https://support.apple.com/HT201084

You cannot remove a child's account from Family Sharing. The child either has to be transferred to another Family Sharing group or the child's account has to be canceled.

Yes, a link to that is included at the end of the document. I was assuming anybody pursuing my suggestion to read the article would encounter it. Apple's documentation contains most of the additional links one needs if one actually reads the primary link.

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Remove a child from your family group

Your child has to remain part of your family group until they turn 13.* However, if necessary, you can transfer a child to a different family group.

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. 

In addition to what Limnos posted see also - Move a child to another group using Family Sharing - Apple Support.

Apologies Limnos for overstepping as I hadn't realized.