Family Sharing

Correct. I know you can't delete a child under 13. Neither can you change their age. From the link you provided:

Be sure to enter your correct date of birth. After you change it to any age below 13, you can't edit it.

It is currently set at 6 and cannot be edited. The person can contact Apple and see if it is possible to get them to do it.

iTunes [Store] Support - Use this form for assistance with account and billing, code redemption, or purchase questions -->https://www.apple.com/emea/support/itunes/contact.html

You were then pointing them to a way to delete an Apple ID entirely from Apple which is unlikely what the person wants to do either because then the child permanently loses everything even remotely to do with Apple such as all purchases.

See >>> https://support.apple.com/en-ca/HT204164

You can't both change his birthdate AND delete him. Which do you want to do?

He can't "delete" a six year old family member from Family Sharing. Just not possible.

He can move him to another family or delete the Apple ID entirely.

My first link was to change the birthdate. That avoids removal in any sense.

Actually, you can delete Apple IDs now, including children you set up.

https://support.apple.com/en-ca/HT204164

I believe the person wishes to delete the child from Family Sharing but this is not possible for a child under 13, hence the request for changing the age setting.

I think you are misunderstanding what the person wants to do. The person does not (I do not think) want to delete the Apple ID entirely, as in have it removed completely from Apple's system, they merely wish to remove the child from Family Sharing. They want to delete the child's account from Family Sharing. The privacy article talks about deleting an Apple ID entirely from Apple's system.

As we have been discussing, you can not. Contact Apple using the link in my previous post and see if they will.