inadvertently redeemed x box gift cards

Hi I got 2 x £20 gift cards for my grandsons, not understanding the instructions thought I redeemed them to a gift card that could be then printed out to gift to them. I now have £40 sat on my account that I will never use. How can I either get the credit
to my grandsons or get a refund on them.

I don't know if this is posted to the correct place

Thanks

:)