Unless your 6 and 7 year olds are WAY more savvy with money than mine, it's going to be hard for them to understand how much $200 buys, especially when they have that many days to spend it.
How about itemizing the things they will want (i.e. classmate treats, sweatshirt, doll, etc.) and telling them they get one of each these. YOU will know how much money that works out to even if they don't. Tell them they can have 1 (or however many) extra snack each day and let them pick. Number of items seem to be easier than money for kids to grasp.
How about a collection? The pressed pennies are great, or magnets, keychains, etc. I think at that age it keeps them more focused in a gift shop than simply wanting everything they see (I still have this problem 30 years later!). They can buy quite a few of them for the same money as one stuffed animal that will get tossed in with the other 50 at home! I lived near DL as a kid, and every time I went, I got another small china cat.
Good luck. I'm sure you will do well because you are thinking about it ahead of time rather than just spending as you go.
PHXscuba