I talked a friend and his wife into camping a weekend at a rustic Michigan State Park near their home. They have two very young boys, and those boys were thoroughly entertained.
This was mid-August. There was a warm inland lake to swim in, two small playgrounds and the easy hiking trails to explore. The campground was full of other young families as well, unlike the state parks with hookups which I've observed are usually older families and seniors.
One of the boys favorite things to help out with was trying to run the pitcher pump for water. It was a bit heavy but they worked together and got it done.
