Another book that's good for the younger ones is Babette Coles' "Mama laid an egg" - which has the parents telling the kids all these funny ways they might have been made, and then the kids set the parents right - there isn't as much graphic detail as the "Where do I come from" so it's a good starting point if you want some detail without too much detail, KWIM...?

BTW I'm all for educating kids early, but a friend of mine did tell rather a lot of detail to her very precocious 5 year old, who realised very quickly that she had info that had playground currency and spent rather a lot of time whispering and giggling about it - so just bear in mind that at different ages they need less/more detail - be guided by your own child's depth of questioning, I think.