HAVE FUN! What you eat will be the last thing yours kids are thinking about when they are at WDW!
I though of a couple tips I haven't seen posted. Most of the quick food options at WDW have "meals", in particular the kids meals. These can actually be bought as just an entree and is a lot cheaper. So instead of a PB&J meal, with chips and soda you can get just the PB&J and use the juice box you brought with you. Another "el cheapo" trick is to split sodas. My depression-era Grandma taught me this. Buy one large soda, no ice. Then ask for two small cups with just ice. Split the soda between the two cups of ice when you get to the table and voila! you have two small sodas for the price of one large.
I think however that you will do fine with the budget you have. You should take advantage of driving down by taking food with you. You probably have things in your cupboard that you could take without having to buy anything. Instant oatmeal, cereal (take some and put it in a ziploc to save space), chips, crackers, peanuts, raisins, leftover Halloween candy, granola bars, peanut butter, jam, coffee/sugar/creamer, juice, stuff you put in kids lunches for school, etc. Also throw in any tylenol, imodium type stuff that you already have as that is REALLY expensie to buy on vacation! Ziploc bags, napkins, paper towels, plastic cups. I did better at my own grocery store with sales and coupons (and we have the highest grocery prices in the US) than at the SuperWalmart. And if it is already at your house, all the better as you won't need any of your budget for it.
I LOVE surprise trips! They are the best as no one has any expectations. It all seems good as they were excpecting just a regular old week and now they are at WDW!
Have a BLAST! You are an awesome mom to do this and I have NO DOUBT your kids will remember this as well as you remember your surprise trip!
Jennifer