Can't create child xbox account

No. Only child accounts can be managed that way if he created an adult account, it can't be converted to a child account. He'd have to create a new account in order to do so and nothing can be transferred between accounts.

You can change a child account to an adult account, but you can't change an adult account to a child account. In order to manage them, you would have to create new child accounts for them.

In short, you can't. If you've created a child account will either have to wait untill it becomes old enough for an adult or create a new account.

This means your xbox account is a child account. If your xbox account is a child account that can't be changed in any way - even changing the birth date - as your xbox birthdate is connected to the first birthdate entered when creating your account. If that
is so, you will need an adult to create an account, invite you to a family group and change some privacy settings for you. For more info on this contact xbox support about your account: https://support.xbox.com/contact-us

If your microsoft account is an adult account you may be able to access that page but any changes will not take effect for real if your xbox account is a child account. If both are a child/teen account that page will take you to a page about adding a child
account to a family.

Create a child account and lock the adult account with passkey or password. This separates the accounts so children can't access payment methods.