The nursery becomes more frightening when the parents realize that the children wish them dead...or at least unable to control the children any longer. Lydia mentions more than once that the roars of the lions seem all too close for comfort. This is a fabulous motif (a feature or theme which recurs in a work of art, music, or drama) which reinforces Lydia's fear of the lions and foreshadows the final scene when the lions are licking their paws just as they did while eating the kill that Lydia witnessed before all while the scarf Lydia was wearing last blows in the wind next to the feeding lions. Ultimately, Lydia's worst fears come true (she and her husband are victims of the Veldt and the lions which live there) and the children are free to operate the nursery walls as they wish...without parent control or interference.
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.