This would have to be my new favourite book. I love it because it is really the only sex education book that you need for preschoolers and early primary school aged kids. It is simple but it literally covers everything you need to talk about with your child.
It covers gender differences, the private body parts, what private means, what touching is okay, puberty, how babies are made, sexual intercourse love and much more.
You could read the whole book in one sitting or break it up into chapters.
You can buy it from Amazon or Book Depository or Booktopia.