500 will be stretching, but not too difficult if you plan a bit in advance.
There are loads of cheap places to eat - and you will only need breakfast / snack / evening meals, as portions are huge, and I struggle to be hungry enough for lunch (can ALWAYS fit in a churro or 2 though
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It's not my 1st choice, but places like The Ponderosa and Golden Corral are all you can eat buffet style restaurants, and I know that The Ponderosa defo take the kids eat free cards. The quality is OK, but it can get a bit hectic as they're always busy. There is a great selection of food. Like I say, not my 1st choice, but my kids LOVE it, as does DH.
It can be expensive to eat in the parks, but if you must, try going to the burger restaurants where there is a separate 'toppings bar' where you can supplement the meal. That way you can share a meal between 2 and it goes a little further (it's quite common, you won't feel embarrassed). Otherwise you are allowed to take in small snacks from outside.
Are you staying at a villa? If so, you've got a great option of going to wallmart and getting breakfasts and dinner in (rotisserie or fried chicken, salad and bread being a common dinner for my lot).
Even if you're staying at a hotel, takeaway pizza is cheap and huge.
When you're sick of fast food, try Sweet Tomatoes, a buffet style salad restaurant. Sign up on their website to get weekly discount coupons.
Pick up the discount coupon books in stores and malls, you will not be judged for using them!
Have a read at the DIBB forums, you'll find lots of advice on food/spending budgets. 