The Soil Science Society of America has published a book geared to kids in grades 4-6. The book features sections explaining how dirt is different from soil, how soils form, soil particle size, living organisms in soil, soil types, and how soil connects everything together. Each section of the book has been developed into a Teacher's Guide - complete with learning objectives, Powerpoint for use in the classroom (with teacher notes), glossary, lesson plan links (if applicable), test questions and/or worksheets, and activities from the web (if available).
Sections (with links to all of the information) are:
The Big Picture - Unit Introduction
Soil and Water Conservation