Adding credit to my childs iCloud account!

I have set up a new account for my 8 year old. I have added it to family sharing, and activated it etc...

My question is about billing on the account. It is set up so that it has to be authorised by me before any transactions can take place.

I get that if I authorise it will come off my credit card, however if he was given a gift voucher, and I add it to his account and not mine, will it take if off his account first, before taking credit from my account to clear any potential balance.

Hope someone can help! I don't want to add credit to his account then