DD knows that it takes a boy and a girl to make a baby - we haven't gone as far as talking about sperm and eggs yet. We have the second books listed above as it was written by a family friend (there are 3 books for various ages). I read it to DD about a year ago and when it came to sex (I think it just mentioned a mommy and daddy loving each other) she didn't ask any questions. She has a 2-mommy family in her class but the boy was adopted, but we've used that family as an example to talk about all different types of families. She knows that she and DS were cut out of my tummy but that most babies come out of your vagina. I guess she told my mom the other day that she was never going to have a baby because some girl at school told her that it hurt a lot to have the baby come out.
We are very open with nakedness in our family, though me more than DH. My parents were the same. DD and DS have some facination about their different body parts, but more facination about what theirs will look like when they are older.
DD has definitely been asking questions when she sees my tampons, but I have pretty much brushed it off - I think I've said a few times that it was so I know i don't have a baby in my tummy and she's never pressed for more information (she has no clue that tampons are for blood) and I don't plan on discussing that for quite a while.