My boys are turning 5 and 9 on our upcoming trip. They will each have $10 a day to spend.(they've been saving Disney Dollars from gifts/holidays, etc.) They can spend it all at once or break it up. That is up to them. But they both know that when it's gone, it's gone. I will hold the money for them. I know how easy it is to lose money, and how heartbreaking it is when you do. I just feel better about holding onto it and giving it to them when they need it.
On our last trip, they were 3 and 7. We gave them each about $40 to spend. It was hard to teach the 3 y/o the value of a dollar. Fortunately, he wanted "little things" that only cost a few dollars each. My older one, though, wanted BIG things that cost too much. I had to re-direct him a few times!!
I'm sure my little one will still have a little difficulty understanding the concept of "when it's gone, it's gone" but we have been prepping him for a while now, so I'm hoping it sinks in! I totally believe in prepping them ahead of time...how much they can spend, when and where they can spend it, what they can spend it on (yes, I do put my foot down on certain items, even if it is their own money!) etc.
Good luck!