I've been to Hershey a few times and have enjoyed railfanning in the area. The NS Harrisburg Line goes through Hershey and as previously mentioned has no schedule, so it could be quiet for a while and then you can have several trains in a short time span. There is a newer style signal bridge so you can get a sense of westbound trains approaching. There's also a local that works the Hershey plant as well.
NS 5619
Here is a pair of BNSF trains bringing a westbound intermodal train through Hershey:
Westbound train
Marysville, on the western side of the Rockville Bridge is a spot I like. Aside from the trains, you're at the bank of the Susquehanna River, and my kids always find something along the shore line to strike their interest.
NS 2736 on the bridge
If you will be in the area on a Saturday at all, Harris Tower, in Harrisburg, is open Saturday afternoons. The historical society that owns it gives tours of the tower. If time allows, I also recommend Strasburg as a place to stop and see. Enjoy Hershey. In my opinion, it's a great family-oriented park with something for everyone to do. Just seems that it's overcast when I have gone.... Which is good while you're in the park.