Mrs. Frisby and the Rats of NIMH, Robert C. O'Brien
This was a wonderful story in my childhood and it's still a wonderful story now. A mother mouse fighting to protect her sick child and dealing with the mysterious rats of NIMH. There are a lot of nice characters: the mother, her friend the crow, the wise owl, and the rats themselves. It has some good child-friendly tension balanced with small victories along the way.
