This link is a succinct summary of the Hohokam. It is a good starting place for learning about them. It also contains links to current landmark sites to visit.
This link contains much more information about the Hohokam, including how they lived, where, and artifacts found. Each subtopic is written in only two paragraphs or so, making it quite accessible to the elementary student.
I chose this link because it comes from a valuable resource we have here in Tucson, Arizona. While the text is quite small and somewhat intimidating, it has information that the other two links do not contain. It describes their daily life in a narrative format that is quite interesting.