I have used this book with k aged kids before:
http://www.amazon.com/Its-Not-Stork..._1?ie=UTF8&qid=1295669996&sr=8-1

Here's another one:
http://www.amazon.com/Whats-Big-Secret-Talking-about/dp/0316101834/ref=pd_sim_b_4

They both seem to answer a lot of younger child questions factually without getting into too much detail. We use these books in our church Sunday school for an 1st grade class. Kids seem to like them and we have chatted about the first one on this site before.

A more detailed and mature book in the same series as the first one is:
http://www.amazon.com/Its-So-Amazing-Families-Library/dp/0763613215/ref=pd_bxgy_b_text_b


Warning: sleep deprived