Last trip, we gave DNiece (age 8 then) and DNephew (age 7 then) a total of $120 each for a 6 day trip - so $20/day. I kept it in separate envelopes in my purse, and before each kid bought anything, I let them know how much they were spending and what they would have left. The first night - they spent about $30 - $40. By the third day, they became very unwilling to part with any money. They actually each had a couple dollars left at the end, because they became much more careful about spending when they saw how quickly the money went. If they were trying to buy something that seemed a waste, we would suggest they "think about it" and we could come back the next day to buy it. They even bought a gift to take back home to their Mom as a thank you for letting them go on the trip with Aunt & Uncle. (We then put an 5x7 picture of the kids with Mickey in the gift frame, and had the kids give it to Mom when we returned them).
It worked out well. Since that was their first trip, I'm thinking that we'll give them a little less spending money this September - maybe $15/day.