Peter Mayle explains the facts of life in a way that children can understand and parents can enjoy. He covers all the basic facts from love-making, orgasm, conception and growth inside the womb, through to the actual birth day. It names all the names and shows all the important parts of the body.
Whilst this book explains "where did i come from" in simplistic terms that children would understand (the reading age for this book is 4-8), I wouldn't recommend simply giving the book to your child without reading it first. Some of what is explained is quite graphic. I actually chose to let my 10 year old daughter read parts of it, while I explained other parts.