My personal favourite provincial park for camping is Killarney, but I just realized I've taken the kids to Pinery instead for three summers in a row. The kids prefer Pinery for one reason: the sand.
Pinery is all about sand. It's sand dune after sand dune and the sandiest beach you'll find in this province. We plan these massive sand dune hikes (don't tell the park ranger), sweat up a storm and then spend all afternoon in the water. The nights are for more hiking followed by the mandatory campfire. Sometimes, if I remember, I even feed the kids.
I want to give a big thank you to Canadian Tire for providing me with the biggest tent I've ever slept in, the most comfortable sleeping bags, inflatable mattress and a chair 10x more durable than the piece of crap Maple Leafs chair I bought at WalMart a few years ago. The tent survived an intense thunder storm on Thursday afternoon no worse for wear, and it was larger than the master bedroom in my house. If you're looking for camping gear, check out Canadian Tire, specifically their line of Roots gear.
Next year I'm booking time at two provincial parks: Killarney and Pinery. And, I think the kids are ready to stretch the trips beyond three days.