Like its name, the cause and effect essay discusses the reasons or causes for something and the outcome of this. In this particular example, you would be looking at the lead-up to her disappearance and murder, as well as these effects on her parents. There are a couple of different ways that you can structure a cause and effect essay, both of which start with an introduction paragraph and end with a concluding paragraph. In the body paragraphs, you can discuss separate causes and effects in each paragraph, or you can discuss the causes first and then transition into your effects in the second half of your essay. Because of the topic, I think the later will work best, as you could focus on the fallout from Susie's parents learning that she was murdered.
An interesting thing that you might look at here is the difference between Susie's parents' reaction. As you know, Susie's father and sister suspected Mr. Harvey, while no one else did. Susie's father and mother also have different emotional reactions—her father is very angry, whereas her mother is more sad in her grief, looking to the detective to uncover answers. Because the book deals with the long-term outcomes of Susie's death, you could also look at the immediate effects, as previously discussed, and the later effects. Later within the book, we know that Susie's mother considers leaving the family, due to guilt and sadness that she feels. Unlike Susie's further, she draws away from her family and her other children. We then see that they do, eventually, come back together and attempt to move on. As you can see, there is a major focus on the different ways that people deal with Susie's death (the effects). I think that an interesting way to approach this essay would be to examine how each event has implications, or effects, that further move the parents along in their healing journey.