Yeah I thought we might end up having to temporarily stick some ply across it or something when we moved in, but it's never been an issue. Our kids are psycho daredevils who don't listen for shit, so I have to assume it's just not that interesting to them. I think the diagonal sheet metal in the hole helps. Effectively, the hole in the ceiling under it is offset by the width of the hole itself, and the metal directs stuff between the hole in the floor and the hole in the ceiling under it, which means that you can't really get a good look at stuff falling down. You can see it go through the hole if you look in at an angle, but not the fall to the floor. The metal itself would be rad as hell to slide hot-wheels down, except it's pretty short and you can't see the car fall on the other side very well so I guess it's more fun to just throw them off the stairs, or the deck, or whatever. Its residing in a small, boring linen closet probably helps.
