In "The Veldt," how does the nursery work and become more frightening?

The nursery works by telepathy. It reads a person's thoughts and then projects them onto the walls. When the story first opens, Lydia feels uncomfortable upon seeing the scene of a lion that is feeding on a recent kill. As a mother, she recognizes that the thoughts of her children are becoming more violent. George, however, just thinks the children have been reading about Africa too much. As the story progresses, more ominous sounds keep coming from the nursery, but George keeps reassuring Lydia that the lions can't cause any real danger. Eventually, they question their children, Wendy and Peter, whose names allude to the story of Peter Pan who refused to grow up. Eventually it becomes clear that the children have learned to program the nursery in a dangerous manner and will stop at nothing to protect it.