Christ's College in Cambridge, England, is where Charles Darwin became hooked on science. The website created by the college to honor Darwin's 200th birthday in 2009 includes a kids' section with information on who Darwin was, what evolution is, and how we know about.
saraband.net
Charles Darwin's The Voyage of the Beagle: The Journals that Revealed Nature’s Grand Plan by Michael Kerrigan is an annotated journal that includes additional material. Suggested by homeschooling parent Jean Watson from Australia, it has not been published in the U.S., but can be ordered online.
Rodale Books
Charles Darwin's On the Origin of Species: A Graphic Adaptation by author Michael Keller and illustrator Nicolle Rager Fuller is a comic book adaptation of the original text, with additional material on the original book itself, Darwin's research, his correspondence with other scientists, and recent breakthroughs in evolutionary theory.
Elliott & Fry (public domain) via WikipediaThis article from the Guardian newspaper of London includes links to a printable storybook, magazine articles, fact sheets, hands-on activities and more. Evolution: The Story of Life on Earth, written by biology professor Jay Hosler and Illustrated by Kevin Cannon and Zander Cannon, is a lively comic book explanation of how life first developed. Although written for older students, it comes recommended by the mom of an elementary-age homeschooler. You can read an except on the publisher's website.