Error message: Check instructions

check the error message you are getting and you can see there the instructions on to where you can change the setting for your child account. if that doesn't work, I guess it's better to contact the support team for Xbox

Yes, I have tried both the Microsoft Store and the Xbox App, both also met with the same error message: "Check the instructions that came with your code. You might need to redeem it on a different web page."

I followed your instructions, but i always get the same error message.

I did that before following the instructions, same error message on Microsoft store

What type of error are you getting when trying to change his gamertag?

Here is a link to instructions on different ways to change the gamertag.

Instructions on how to change gamertag

If you followed the instructions on how to change the gamertag from this link and are getting an error message we would need to know what the error message is that you are getting.