I live in Central Pa. Basically smack dab in the middle of where you are going.
Hershey - It takes all day to do Hersheypark. The park is huge with alot of rides. Plus lines are very long sometimes. But they do have a very nice waterpark inside Hersheypark for kids of that age. Also on the grounds is the Giant Center, Choc World, Hersheypark Stadium and Zoo America. Check for activities in them also. Choc World and Zoo America is a must for kids. There is a campground right close to the Park. I believe they shuttle to the park also. Oh, Indian Echo Caverns is close to Hershey also. That is cool to see.
Gettysburg - Gettysburg has tons of campgrounds. Our favorite is Drummer Boy Campground. The stuffed animal place that was mentioned earlier closed it's doors in 2010. You can drive thru the battlefield and let the kids explore Devils Den, Little Roundtop and the Peace Light. There are also Ghost tours, Museums, Gettysburg Harley Davison, Land of the Little Horses, Horseback riding tours, the National Cemetery, Visitor Center, and the Explore & More Children's Museum.
Lancaster- Amish country also has alot of Campgrounds. I believe there is a KOA over there. For young kids, Dutch Wonderland is a better option than Hersheypark. There is also Strasburg Railroad and a National Train Museum close by. (I know they advertise Thomas the Tank Train at different times on TV being over there) But you can ride the old steam train at the railroad. Turkey Hill Experience is over there also.
There is also outlet shopping in Lancaster and Gettysburg. The
Pennsylvania Renaissance Faire is in the fall or late summer.
Personally I would find a campground in Gettysburg or Lancaster. Check out (all in Gettsyburg) Granite Hill Camping Resort, Gettysburg KOA, Artillery Ridge, Drummer Boy, Gettysburg Battlefield Resort, or Roundtop Campground. I don't know Lancaster as well as Gettysburg (I hate the roads and traffic over there)but you got Old
Mill Stream Campground (at Dutch wonderland) I hear good things about all of them. Granite Hill is well known for its Summer Concert Series. Also don't forget we are only about 1 and 1/2 hour from Baltimore. And there is the National Aquarium, Inner Harbor, The Historical Ships, and the Science Center.
There is a Thousand Trails(use to be Outdoor World)Campground about 5 minutes from me. It is in the middle of nowhere, but all these places are 30-45 minutes away. It is a membership campground, but non-members can camp also if sites are still open. Alot of ppl go there also.