I have looked around a very very SMALL amount. If its a play structure like the 1 or 2 room, 1 level houses for field play. You do not need a permit. Things like small sheds depending on size for storage you dont need a permit. Add a tent or some meshed area as safe zone and you could potentially get around most your building permit needs.
Again just from my limited looking, I'd look into detail with the township a little more about that.