Kids can trip over mushrooms and not notice. Whenever you see a mushroom, start hunting for a kid, pick him/her up and drop him/her on the mushroom. Hopefully they'll make it all the way to the food bin with the mushroom intact. However, I've had instances where they get stuck on a tree root, building, or even the "wrong" side of the food bin itself and "poof", the mushroom is gone. It's REALLY nasty when they lose a red mushroom!!! When the game says that the children are "untrained" they mean it!