change child's account region

In all likelihood the region for your child's Microsoft Account is set to the US instead of UK.

It's quite possible for the region for the Microsoft Account and Xbox LIVE account to be mismatched.

To check the region for your child's Microsoft Account, log in to
http://accounts.live.com using your child's Microsoft Account email address. If you change the region for your child's Microsoft Account, you should have no further issues.

If you're unable to change this, then you may need to contact support by phone to get it fixed.

Sounds like you will have to change the region of the child account. Here you go support.xbox.com/.../change-region

For most people, in most regions

Sign in to Your info on the Microsoft account website.Select Edit your personal info.Change your birthdate, and select Save.
For child accounts, in some regions
If the birthdate on a Microsoft account says that it belongs to a child, there are different steps to take in regions such as the United States of America and the Republic of Korea, and other regions that regulate what children can do online.
I'm a parent, trying to change my child's birthdate

Using the parent's Microsoft account, sign in to your Family page on the Microsoft account website.Select Manage my child's profile info.For the child’s account that you want to change, select Edit this child's personal info and then follow the instructions.
I’m an adult, but my account thinks I’m a child
To change your own account from a child account to a full account, you need to prove that you're an adult. You have two options:

Show all
Option 1: Provide a valid U.S. credit card number
Option 2: Send us proof of age

Might I be able to change it a different way

Hi Martin G, my name is Courtney and I would be happy to help.

We really can't imagine how important it is for you to be able to update/change the country or region of your child's account as soon as possible. However, we have realized that this issue of yours would be better addressed by our live support team so that they can properly check what is going on with your device and its apps. They are more equipped with all the tools and resources to diagnose your problem.

Support: https://support.microsoft.com/home/contact

Information source (Credits): https://answers.microsoft.com/thread/2567020c-eb49-4d1d-b4c2-e286444e1cc6

I hope you understand, I wish you good luck.

Best regards,
Courtney.

Hello Martin, I'm sorry to hear you are having this problem!

I'm Olly, an Xbox Ambassador, and hopefully I can point you in the right direction!

Click this link to go to Microsoft Support

To address your concern, navigate to the search bar and enter your issue. Press Enter or click "Get Help." Sign in to access support options. Choose the relevant product, select the appropriate category, and confirm your selection. The system will provide you with support options to assist you further.

Try and get into Live Chat, where a Microsoft Support Agent can change this for you!

Best of luck!