How do I change my kid account to an adult account or remove the restrictions

You will need your parents to change the settings. If you have reached that old "adult" age, then your parents will need to remove you from the "family" account.

I assume this an ongoing issue still? Just tried to make a Games with Gold purchase and its blocking me from making the purchase an a rating category. Thing is I'm signed in my actual Adult account. Yes I have 2 kids accounts with restrictions, but removing,
re-adding, changing restrictions etc doesn't resolve the issue.

The problem with this is: Since your account is an adult account, even if you could put time restrictions on that, your kid using the account would be regarded as an adult thanks to the adult account, and the kid could just go to the account settings themselves
and revert the changes.

That's why child accounts can't make such settings and require a separate adult account to do it.

Let's make a real world analogy:

You as a parent could go to a bank and tell the cashier that your kid has a "child" account at their bank, and you want to limit the ability to withdraw money to 100$ per month.

Since the kid is unable to sign contracts and stuff, if the kid goes to the bank and says "I want the 100$ limit lifted because I want to buy Disneyland!", the cashier will say "Sorry kid, due to your age you can't make or change contracts. Get your parent
here to lift the limit."

If you as an adult go to the bank and say "I want to limit my spending to 100$ per month", and a few days later come back and say "I as an adult want to buy a boat, a house and a new car, so lift my 100$ spending limit", the bank will comply as you are the
adult.

So unfortunately, you can either leave the kid unable to be restricted, or create a new child account where the kid can then be restricted, but loses their game progress.

Also be aware: The older the kid gets, the more they want their own account. And the more YOU want them to have their own account, as the older kid probably then could see your emails and use other Microsoft Account services on your account.

So personally, I can only recommend to create a new account for the kid, and have them replay all the fun games again. With time restrictions, if needed.

My son has an adult microsoft account with child restrictions on our xbox one.

...

What would happen if he deleted my parent account? ... And is there any way to prevent him from creating a new account just to get around the restrictions? Its impractical for us to create a passkey to the console because we arent always around. I
just want to prevent the creation and deletion of any account...

First of all: You say your son has an "adult account". Adult accounts can be set up to be restricted, however as the system sees the account as "adult", the owner of the account can simply set these settings back.

I assume you just mistyped, and your kid has a "child" account with restrictions you set through your adult account, right?

As for closing your loopholes:

Go to the Settings of the console, and under "System" choose "Control offline content". Set the setting there to prevent creation of new profiles.

If your parent account gets removed from the console, it doesn't matter.

Online and Privacy settings are managed online on the Live service, to prevent your kid going to a friend, signing into their console and thus "circumventing" your settings.

With the settings being managed online in the "Live cloud", no matter where your kid signs in, the parental restrictions will go with your kid, wether the console has your parent account on it or not.

As long as your kid's child account is added as a child account to your parent account, they can't change anything.

And it's recommended that if your parent account stays on the console, that you secure your own account with a passcode, that your kid doesn't know.

That way the kid is "forced" to use their own "unlocked" child account with the parental restrictions, instead of simply using your parent account to get around the restrictions.

the same problem is happening with me. i removed the adult account and turned off the parental restrictions and it still says i am a kid and that i cant do a lot of things because im underage, which im not