Um, i vote for the disney gift card vs the room card! At least if you give them the gift card, they have full responsibility over their money, so if they know that once they run out - they run out, maybe they will think twice about wasting their money..
Now, if they carry around your room key with charging on it........ hmm... think of the possibilities!! no thanks! LOL
I think the gift cards are GREAT.. You could even go as far as to give them one a day - with a set amount of money on it, and then they know that is their spending money for the day.. period.
I myself used the gift card system last trip, i put $500 on one for strictly food, and then carried another for spending money.. it worked out great, and will be doing that again next yr.