Nixon Park is my 8-year-old daughter's favorite park in the area. She brings a net, a critter-catcher container and has a blast in the shallow stream. Wear water shoes, because it can be muddy) She catches dragonflies, butterflies, little fish and crayfish. (Catch and release only -- of course!) And the nature center sometimes offers lessons about things like salamanders, or reptiles, but you have to check their calendar.
Great for hiking and learning, but no playground for littler kids. Around the Jacobus area, Nixon park is out of the way, and a beautiful area. Trails can take you up the steeper hills, out to a meadow, and through the wooded areas. There's a creek that runs though, and a garden area with a path to view plants and butterflies. There is also an indoor nature center with many aspects for children to learn.