GLOBE's Soil Module storybook and learning activities contain a wealth of background information on soil. The Scoop on Soils, a child-friendly picture book on soils, is downloadable as a PDF file. The site also has a few simple soil activities with worksheets.
The Environmental Monitoring and Action project has an excellent page of resources on Soil and Land Cover (Biodiversity). The Going Underground- Visual Soil Assessment curriculum planner is comprehensive.