Accidentally downloaded prepaid code to wrong account

You would need to contact support
https://support.xbox.com/en-GB/contact-us as they are they only ones who can help you with this.

They should be able to cancel the gold on your account & give you a code to redeem on your sons account.

Please head over to commerce.microsoft.com to ensure the code was activated on your account. If it was not perhaps you entered the code on the wrong account accidentally.

If the code was not accepted to your account or you entered it on the wrong account accidentally you should give phone support a call for more assistance. 1-800-469-9269 in North America.

Hello Mario,

Thank you for posting on the Xbox forums. We understand that you redeemed a prepaid code for Minecraft, but you are unable to download the game. Is this issue occurring on an Xbox One, Xbox 360 or Windows 10 device? Is the prepaid code for the full
game or content for Minecraft? We recommend checking the
license list for your account, and any other accounts that you own. It is possible that the prepaid code was redeemed on the wrong account. Be sure to let us know the results!

Hey there Supercrazysquid

Did you redeem the code on the same Microsoft Account? There may be a chance you were signed-in to a different account

If you did accidentally redeem your prepaid code on the wrong account, I recommend reaching out to the Xbox Support Team. They will be able to assist you as long as you have
your prepaid code. We will provide steps on how to contact the support team. Be sure to let us know the results!

How to contact the Xbox Support Team:

1. Click the support link that we provided
2. Select Billing & accounts in the category area
3. Select See other options
4. Select Can't redeem a code
5. Select Show contact options
6. Select Request a call from Support

Is there any chance you accidentally redeemed the code on the wrong account? This would be the only reason why the code would redeem but you cannot find it on your account afterwards.