We just went to the public pool @ Marsh Creek state park about 2 weeks ago and it was fantastic. It was $6 admission (all ages) and they had a pool and splash park. The splash park had enough going on for little & big kids, and then the pool itself had a shallow "end" which was an entire length of the L shape that started at about 2ft and went to 3.5. My 3 year old twins are pretty small for their ages and were BESIDE THEMSELVES that they could touch the bottom in the "big pool."
Additionally, and this is only something I realized was a problem this past week - they allow swimmies/water wings in the pool area. I had no idea that a lot of places in the area don't allow ANY inflatables or swim aids outside of the toddler/kiddie pool.
You can get your hand stamped if you leave and want to come back later, and the park has picnic areas & a playground. There wasn't a LOT of shade, but there was some around the perimeter of the pool fence. I dont' know if you're allowed to eat in there, but we did, and nobody said anything.
And the biggest bonus is that it's not too far away from the Chester Springs Creamery - for us, it was worth the drive (a half hour from where we are in Delco) and it ended up being a full-day event. PLUS, for me, with three kids under age 5 + being 30 weeks pregnant, that I was able to do the whole thing myself + not be completely wiped out at the end of it felt like a huge accomplishment!