In all fairness, YOU put the wrong age in the field and finally Saved the form...
in the frist linked article, it says:
To change your birth date to any age below 13*, you must be a member of a Family Sharing group, and your family organizer must approve the change. After updating your birth date, follow the onscreen steps to send an email request to your family organizer. Your family organizer can sign in to My Apple ID to confirm the birth date change and agree to the Parent Privacy Disclosure agreement by entering the CVV of the credit card on file in their account.
Be sure to enter your correct birthdate. After you change it to any age below 13, you can't edit it. Learn more about Apple IDs for children.
This means, to solve your problem, you need to create a NEW Apple ID for the child, add that Apple ID to the Family Group. The end.
You may feel free to try Contacting Apple for help with Apple ID account security - but the rule is there for a reason = to keep kids from "magically becoming an adult" for purposes of all that can be done as one at Apple.