Edit: Sorry, just realized this is in the cruise forum. I was answering in a WDW perspective. I think the gift card would still be an option, but you obviously won't be making any phone calls to re-load it.
Don't turn on room charging for their keys. Get them a Disney gift card instead that you can load with the amount they're allowed to spend. If you don't want to give them the whole amount up front, you can also call the number on the back to re-load the gift card using a credit card.
If you want them to use the room key, you won't be able to cut off their spending at a pre-set limit, but you can check what they've spent. The front desk can print you out a listing of all charges. It shows which key made which charge.