Figure out how much you NEED (for tips, TS gratuities, parking, etc..). Then, determine how much $$ you could allocate for each person to spend on souvenirs. Last year, we let the kids (then 7 and 4) have $75 each... and that sometimes seemed too much.
Take a look over at the Restaurants Board to see the information about using snack credits and counter service credits wisely. You can often use snack credits to purchase items you wouldn't normally consider (say, a cinnamon roll for breakfast). And counter service credits for adults and children are all one kind of credit (at least, they weren't last fall)... so the kids aren't limited to children's meals for counter service. You could get an adult-sized meal and split it between the two younger kids. And counter service breakfasts include a juice instead of a dessert... so if you use CS credits for breakfast, you could get your meal, a bottle of water (to take for later) and a juice. Honestly, you can save a LOT of money by using your credits to the best advantage.