This is a really informative book, without going into too much graphic detail, making it incredibly useful in helping explain bodily functions to children.
I have found it invaluable with my five year old son and we often refer to it when he asks us "tricky" questions about his body. It's also useful to know how much information is appropriate for his age.
We would definitely recommend it to friends to help with their children, and have even given it to parents we know as a gift when their child reaches a suitably inquisitive age!