My account is stuck on child default (it was set as adult when being created)

create another adult account on your xbox and use that to change your current account to adult as well

If your child is not an adult account, being a child account when created, and never promoted to an adult if you are of age, then it will require the parental email to sign the updated TOU.

There is no way around this.

[...] It says I need to change Others can communicate to everybody, with a link to the page. However, when I click on the link I have been redirected to a change settings on a child account page. I have never known of any "Adult" account being linked
to mine [...]

That message about child accounts usually happens, if you are signed in on a child account (i.e. an account that is not yet of legal adult age according to the birthdate you have entered when you created the account).

If you have a child account, you will need an adult parent account that signs in on
https://account.microsoft.com/family and adds your child account to their adult family account.

Then the adult has to go to
https://account.xbox.com/settings and click on your child account. Then they can set up your child account's online and privacy settings accordingly.

Its an adult account ....

In that case you already do NOT have any control over the account anymore.

As soon as an account is deemed to be an adult account (determined by the birthdate entered when the account was created), the account can leave the family, and set their own online and privacy settings.

There is no way to control an adult account with another adult account.

Only child accounts can be controlled by an adult account. And if your kid is using an adult account, they are regarded as being adults.

The way this is supposed to be set up is that you have a child account and an adult account. The child account is a separate account but it cannot act independently of the adult account. From the adult account you can set purchasing permissions and even
set a monthly allowance that will not be exceeded. You can also set restrictions on what you child views or sees and how they can pick friends.

The problem is that most of the time when these accounts are set up, parents do not create a child account, they just set up an adult account for the child, and we cannot set permissions on an adult account.

The only thing you can do with an adult account is remove any payment methods and add gift cards to the account as needed. That way when the money is gone its gone.