Henry's Freedom Box: A True Story from the Underground Railroad - Ellen Levine
First off, I *love* that cover illustration. It might be my favorite ever. This is the story of Henry 'Box' Brown - the slave who literally shipped himself to freedom after his wife and children were sold. Told in simple prose and accompanied by luminous artwork, this book makes it a little easier to discuss the concept of slavery with young children. Some may be disappointed by its brevity and of course it's not the full picture, but I think it's a good start!