I'll also second the motion on Knoebel's Grove (in Elysburg, PA). A great old-fashioned park that charges no admission fee - you pay only for the rides you want to ride, so if Grandma & Grandpa just want to sit and watch the kids have fun it doesn't cost you anything. Food is good and reasonably priced. There is a CG attached to the park but it is electric only. There are some full service CG's nearby.
Hershey Park is nice, too, very modern and more expensive.
We also liked the Jellystone Park near Luray Caverns. We no longer patronize Jellystones (long story) but there is stuff for kids to do right in the park, including a small waterpark. If you have time for a good day trip from the Luray area, drive over to Cass WV and ride the Cass Scenic Railroad. Enjoy & safe travels.