They trust everybody. They do not see or look for sinister plans or plots. Most, if not all, plots are based on the interaction and blocking of some characters' plans as they strive to achieve their goals. Tracking such things in detail is often beyond a child's experience. He must learn how.
Movies can be an aid here.
Children who watch movies will learn something of plot development and sinister plans. But there is a great difference between processing text and processing pictures. In reading, many more inferences must be made about what characters actually have done. In movies, actions are spelled out in visual detail. Understanding that a character has a plan, and inferring the details of his plan is easy when watching a movie because we just watch the plan develop.