I can't change my child account to an adult 1

To verify, a child account can't change their birthday, but an adult account CAN change their birthday if they so wish. I CAN change the birthday that's associated with my account, I CAN'T change the birthday associated with my daughter's child account.

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.

First question, are you using a child or an adult account.

I ask because if it's a child account, you can't change them without an adult account

I can't remember 100% but I am sure whoever set up the child account can change it. Like if you made an adult account on the xbox before making the child account that adult account can change wether the child account is a child account or and adult one.
I will just test this out myself now and post how it went

The "adultness" of an account is determined by the birthdate you entered when you created the account.

Only if your account is at the age of legal maturity in your country, your account automatically becomes an adult account.

While you can set your account to have "adult" settings like allowing you to talk to everyone on Xbox Live or play Mature or Adult Oriented games, this still does not unlock the feature of changing your gamer picture.

That is only possible on "real" adult accounts that reached legal adult age according to the accounts birthdate.

And no, you can't change the birthdate afterwards.