Mrs. Frisby and the Rats of Nimh, by R. O'Brien.
We're getting close to the end. It's been a big hit.
Mommy mouse wanting to protect her family = major appeal to Pumpkin, who loves animals SO much. She is so concerned about poor little sick Timmy. His mouse house needs to be moved so that he can make a full recovery!
Rat civilization & infrastructure = major appeal to Superboy, who loves anything having to do with the design/structure of a particular civilization. His 4th grade class had a massive study of ancient civilizations last year, and it really got his wheels turning. (Ancient Mesopotamia, Egypt, Greece--each one was covered throughout 4th grade.) In fact, his favorite recess game to play right now (5th grade) is some sort of miniature civilization that he and his friend Grant build on every day. I need to ask him for more details---sounds interesting.