While it doesn't happen often, nets have been known to fall over and severely injure kids who do this. Yes, the nets are supposed to be anchored, but sometimes they are not, and sometimes the anchors are insufficient to withstand the torque of a person hanging on the top post.
So, today as I was checking in the little 10 year old boys, I saw one of them jump up and hang from the goal. I directly, and within hearing of the others, told him to get down and to never do that again. Three minutes later, one of his teammates did the same thing, a clear demonstration of the sound-brain barrier that exists in little boys. I decided it was time to make sure none of them would ever forget the lesson: In very strong and loud language, I stood just a few feet away from the second boy and looked him in the eye and told him that he should never, ever do that again. He was cowed, as were his teammates. And then I said to them all that if they ever saw anyone doing that, they should tell him to get down.
Then, gently, I said, "Now go and have a great game and have fun." And they did.