How to change my child Apple ID account from adult to child

I want to change my child account type from adult to child.

My daughter Apple ID account was created as an adult by mistake, at the time I was not aware that a child account was even an option.

now when I want to use Apple family sharing it identifies her account as “adult”.

ive tried to change her date of birth on her iPad but a message show and asks me to visit https://gsa.Apple.com and sent a request to the family parent (me) but the link is not working (showing as inactive sub-domain).

what should I do?

abandon the original account and creating a new one is not an option.