You guys all have cooler dollar stores than the one by my house. I went pre trip and all they had that I found useful was rain ponchos and some coloring books for the trip.
Snacks are good, especially if you can bring something semi nutrious, like raisins and nuts.
Don't forget hats and sunscreen for small faces. Both will set you back bookoo bucks in the parks. So will diapers, children's tylenol, etc. So lug in that sort of stuff.
Don't take the preschool set (you know, the age where they are still too young to have their own budget, but old enough to want) into the stores. Just drive straight past them. Distract them when near souvienier carts as well. You won't need to say no, and you won't need to spend money. (I have a cousin that always shocks me by complaining how much she spends when the kids go with her to Target and want toys. Duh. Two answers, don't bring them near the toys at Target (don't bring them into Target if hubby can watch them) and learn to say no!)
Teach your children early that splitting food is what you do on vacation. Dole whips are huge, just get one and four spoons! Besides, that way you'll save room and money for a Beaver Tail later on.