Eastern PA-July Resort Style waterpark & Kids activities

I'm fond of the Knoebels campground. The campground itself is nothing special, but the park is wonderful and does have a (small) water park area and a whole lot of other very family-friendly activities and attractions which can be enjoyed without sucking one's retirement savings dry. The Knoebels campground is adjacent to the park, up the hill from the roller coasters. There are other (and frankly nicer) campgrounds in the general vicinity, too, but not within easy walking distance of the park.

The HersheyPark campground might be a reasonable option as well, though it's more western and less central.

Edit: looking at a mao again, I guess Hershey is about as central as it gets. My geograohy was a bit off.