International Rock Flipping Day is Sunday, September 20th. Turn over a rock, see and identify what's under it, and write a blog post or submit a Flickr set of photos. (And put the rock right back where you found it.) It sounds like something a six year old invented, but the credit goes to several grown-up nature enthusiasts, including one "doyenne of invertebrate bloggers." You'll find more information at Wanderin' Weeta, a nature blog.
Junior and I participated last year and found Asian shore crabs, mussels, and clams under rocks at the beach. See this report from September '08.
Dave Bronta, a flip-day founder who blogs at Via Negativa, wrote, "[P]eople flipped rocks on four continents on sites ranging from mountaintops to urban centers to the floors of shallow seas. Rock-flippers found frogs, snakes, and invertebrates of every description, as well as fossils and other cool stuff. As before, we advise wearing gloves for protection, and getting the whole family involved — or if you don’t have a family, rope in some neighborhood kids."
Five, six, and seven year olds interested in the rocks themselves might like Nancy Elizabeth Wallace's Rocks! Rocks! Rocks! (Marshall Cavendish Children's Books, 2009), a picture book that introduces concepts about rock formation and other topics in simple language. (I wish the author hadn't cutely called igneous rocks "iggy," but I doubt kids will care.)
For more in-depth reading, older children can look for National Audubon Society First Field Guide to Rocks and Minerals (Scholastic, 1998).
Review copy of Rocks! Rocks! Rocks! provided by the publisher. The Audubon Society field guide from our home collection.