how can I top up my childs icloud storage if he hasn't a credit card linked to his account?

I need to upgrade my childs icloud storage to a larger amount, because they use it for school work, but he hasn't got a credit card linked to his account. I have a separate apple ID & itunes account linked to my credit card, & gifted him some money by a virtual itunes card, but of course he can't use it. How can I increase his storage, please?

Thanks, Andy