“ Questions children ask & how to answer them” by Miriam Stoppard, DK
The special part of this book is the answers for different age group clearly listed out. I've tried to explain my two kids about sex, vagina and penis ... with the reference of this book. In fact, don't try to say too much until they asked. But once they start to question about sex, it is a MUST to give them clear answer and positive attitude, so they won't feel shy to find sources in the net or from fiends.