Family Sharing and Apple ID for your child - https://support.apple.com/HT201084
Leave [/Cancel] Family Sharing - http://support.apple.com/HT201081 - How to leave a family sharing group. How to cancel it or remove anyone over 13 if you are the organizer.
https://support.apple.com/HT204396 - "The iTunes Service is only available for individuals aged 13 years or older or children under 13 who have received an Apple ID through the Apple ID for Students program. If your child is under the age of 13 and isn't an educational Apple ID holder, Apple must close and delete all information associated with their account."
So it looks like you could delete the child but you would have to get Apple to do it as part of their deleting the child's account (losing all purchases in the process).
It is unlikely Apple will change the credit card requirement. I would guess it is integral to ensure Family Sharing be done between people who are part of a close group and prevent it from being any college dorm somewhere.