Here's what we did at
Disneyland 2 years ago and what we plan to do on our upcoming trip. I will buy everybody a snack or two a day and by the end of the trip, I will buy everyone one or two nice souvineers (a t-shirt, cap or something). Beyond that, each kid gets $10 per day that they can spend on whatever or save.
Here's how it worked out at DL:
DS (then 12) used most of his money for extra churros, ice cream, etc.
DD (then 10) spend every cent and used her allowance, too
DD( then 7) bought a Crocodile Hunter cap at Universal and took the rest of her money home.
It's amazing the things kids learn when it's "their" money. When they asked for ice cream (when I had already bought ice cream a couple hours before), I could say, "Sure, but you'll have to use your money." Some times they went ahead and bought the ice cream and some times they didn't.
It's a bit heartbreaking to see the kids realize that even though they have what seems like a lot of money, it's not enough to buy a sweatshirt and a cap and a t-shirt and a jacket and a toy....But it's good for them to learn to set priorities and budget.