According to the Writer's Almanac, today is the day in 1831 that Charles Darwin set sail from England aboard the HMS Beagle for his famed trip to the Galapagos Islands. That trip and his forthcoming book, On The Origin Of Species, became one of the great milestones in scientific thought.
There are two books for kids on Darwin that I highly recommend. The first is Peter Sis' book, The Tree of Life. Published in 2003, it follows Sis' other beautifully illustrated and well researched picture book biographies. You can see an animated excerpt of the book here.
The other is Deborah Heiligman's Charles and Emma. Unlike other biographies on Charles Darwin, this book switches the focus to Charles' relationship with his wife, Emma. Their marriage greatly influenced Darwin's scientific work. Emma was a deeply religious woman and Charles was exploring theories that would greatly change social and scientific thought forever. The book is well-researched and is a great read for both adults and kids.
There are great resources to accompany these titles for classroom use. Discussion questions, teachers guides and book group materials can be found online. Both afford students and teachers great critical thinking opportunities and work well with core curriculum standards.