I had a similar experience at the sprinkler park the other day; Jack and another toddler were happily playing with one of the sprays when a 6-7 year old girl came and sat on it. "They can't play with it if you sit on it," I said, and there was a near-stalemate, but then she moved enough that there was some spray for the babies. And when I told my sister about this (she has three teenage girls), she told me that you just have to tell the other kid to stop, or give it back, or move, or whatever. You can't be polite. Because what is the goal? To teach a lesson or to get your son his toy back? It's very difficult for us super-polite people to do, but when it comes down to it, you just have to say, "Hey, that is Jack's bucket," and then TAKE IT and give it back to Jack. Just like I should have said to the girl, "Don't sit on that, these two are playing with it. You have to share."