H.P.
"It's So Amazing" by Harris and Emberly. My daughter is 9 and son 6 and we got this for them (although daughter is more interested). We have taught our children (including our 4 year old daughter), about sex in a scientific manner. Although some parts of the book can be uncomfortable for adults, children do not judge unless they are taught to do so. We tried to present the information without any particular tone. Recently, my daughter has asked about the logistics of the whole thing and this is when we started discussing the appropriateness of sexual intercourse, age etc... There's also a book by the same authors I think called "It's Perfectly Normal." This deals in more mature matter and also discussed grooming and the like. It's not suited for younger ones (younger than 8 or 9). I have found it is best to answer questions accurately rather than "putting off" the inevitable.