Let them help you build the fire in the fire-pit, it gives them a great sense of accomplishment, especially picking up kindling and small sticks. A scavenger hunt with easily found items like a bird, a pine tree, something soft, something brown, a dog, etc. Kids love bubbles and you can buy them for next to nothing at the dollar stores. Cheap flashlights for them to play with after dark are priceless, as well as chem-light necklaces and such. Can you bring their bikes? Walks around the campgrounds are always entertaining. Don't be afraid to let them bring a couple of their favorite toys for the times when the adults want some quiet time or are preparing meals. Having a few special toys that only go in the RV is nice too.
Doug