The Wonderful Way Babies are Made is one of the best books I have seen for helping parents explain the facts of life to their children. It discusses babies, the sexual act, and reproduction all in the context of families and God's plan. It also devotes some attention to adoption, using Jesus as the example of an adopted child (adopted by Joseph).
An unusual feature is text written on two different levels. On one page the text is written for young children (up to about age eight). On the facing page is text for children about nine and older that provides more detail and information.