Our trip of Feb/06 we did a variation of this with our kids then 4,6. We encouraged Disney or US $$ as part of Christmas gifts as they always seem to get a ton of toys. The kids knew how much money they had to spend and we set up a few ground rules. As we were camping, we just took a bit of money with us each day and locked up the extra (you could use the hotel room safe to reduce risks of carrying lots of cash). We told the kids that there was "x" amount of free choice for them - but they couldn't buy stuff on the first day to a park (that trip was for 2 weeks, and we hit every park more than once, so we could do this). The rest had to be discussed with us. If something really special was the purchase for the day and was over the amount they (me in safekeeping) had, they could borrow from us and pay us back at the camper. I kept envelopes with a running tally for each child and receipts to help keep track. We talked a lot about meaningful souvenirs, what would represent the trip for them at home, etc. My kids are big cuddly fans, so we limited purchases to one stuffed item only. They also had to use their money for Mickey bars and other treats (which of course by the then of the trip was all from us). The intention was to get the kids thinking more about what they wanted and why. Since Disney so cleverly ends everything in a gift shop, it helped a bit with the constant "Oh, I want that!" We had our 7 year old neice with us and this really helped with her - she is a major shopper and really came to understand the value of money and meaningful choices.
If your kids are responsible enough for smaller amounts of cash, come up with a system where you pay for larger items and they give you an IOU to pay you back at the end of the day from wherever the bulk of their money is. If my kids had larger amounts (even at your kids age), I would probably want it in a safe place - that is my comfort zone for responsbility and risk - think about yours and what you want to do. You know your kids best and think up a system that will give them some personal choice and responsibility as appropriate that still addresses any concerns you might have over keeping the money secure. Having a system in place where the kids are responsible for their own snacks, (what, how much), per day might help with the constant reaching into your wallet! They think about what they really want to eat, for example, not just 'cause that ice cream back there looked yummy, but because they really wanted to save their own snack money for popcorn during Wishes....
There are lots of ideas on the Dis...I'm sure more will chime in!