For this grade level I would recommend a series of books developed by the American Geological Institute in association with It's About Time, and  EarthComm http://www.its-about-time.com/index.html

 The curriculum is inquiry-based, and makes a strong connection between the science content learned in the classroom and real-world applications. At this grade level the stronger connection between what they are learning in school and how it is connected to their own lives, the more likely they will be interested in learning about science.

Good luck!

-Julie