Living in Hershey, I'll underscore some already mentioned options. Middletown Reservoir is a great place to go, but much easier with a young one after the first couple weeks of trout season. It gets quite a lot of pressure. Mid summer blue gill and bass are almost ready to play. The pools below the damn are signed for kids to fish during trout season.
Dauphin County Anglers association's coop nursery is bounded by a kids only (12 and under) section of the Clarks Creek in Dauphin. Great place for a picnic, seeing the fish in the pens, and bonus that 3B Ice cream is nearby.
Bullfrog valley pond by the Med Center has a few willing sunfish in the summer. Early May Derry Township has their trout derby. It is posted once they stock it until the derby. It is a total zoo, shoulder to shoulder early in the day. But still a nice event. I'd suggest going there later in the day when the crowd disperses.
Manada Creek is the closest stocked creek. I can recommend a few spots.
All that to be said, when I take my little nieces and nephew out, we dig up some worms and go to Middletown Reservoir.