I haven't read all the posts (Maybe 1/2 - 2/3?), but here is how we handle it:
We decide what we are purchasing upfront and let the kids know that. This might be shirt/hat, autograph book, pressed penny book, the foam dinosaurs, whatever we decide on.
Then we give the kids either a $ amount or # of items that they can pick out through the week. They can either budget the money or save up and decide what things they want most. Which we choose just kind of depends on how much we have budgeted for "stuff".
Also, we do pressed pennies. I save the good pennies and quarters in a jar between trips. When those pennies and quarters are gone, so are the pressed pennies they get. It is usually a sizable amount, and with 3 kids that is needed. They tend to pool their pennies and split them up at the end of the trip. This is wonderful for keeping the "I wants" away, as they feel like they are constantly getting something. Really great with the littler ones.
Another way to avoid the "I wants" is to take advantage of the free things--Epcot & AK Kid stations for example. Not only are they having fun doing something, but they are walking away with things.
As far as snacks go. That is part of our food budget and we pay for that. We are not going to make the kids pay for their Mickey bars or popcorn. We do limit how much junk they get, but that has to do with making healthy eating choices v. money. Yes, we spulrge on vacation, but we don't just jump out of the plane without a parachute either.
Other than that, if they want something, they have to use their own money. They get to make the choice of how to spend their own money period. Yes, we take a portion for charity & into savings to teach them money management. But part of money management is choosing how to spend the rest any way they want.
ETA--some things are reuasble every trip (ie, mouseears, Goofy hat, certain toys). We also have the rule that souvies must say the name of the place on them, with a few minor exceptions. I will not buy something I can buy at
Walmart are half the cost. Or something that could have come from anywhere.