The reason is because Wendy and Peter were thinking about lions in a svannah and those are the images that were projected on the walls. The children wanted to experience lions eating their parents. The father of the children even said that the children used to think of a cow that jumped over the moon and that was projected on the walls. Next the kids pictured a girl sitting in a forest clearing and that got projected on the walls. That was because they were hiding the truth of what they wanted to experience.
