We're going to Hershey Park tomorrow, but haven't been in years. We've never been to Dutch Wonderland because my kids are older (now 4, 10, 13, and 16) and my youngest is a daredevil and wouldn't be happy on only kiddie rides. With the ages of your children, I'd do both! Between those two if I had to choose I'd go to Hershey Park.
But, one of our favorites is Dorney Park in Allentown. There is something for everyone along with a little water park with a great water play area (like the Curious George area at Universal, but better). We went last summer and wanted to go back this summer, but we're not going to make it. We did go to Kennywood and with this Hershey Park trip we are doing ok...
T&B