Okay, so when our DS was about 5 he was HUGELY into human anatomy. It stemmed from one of the magazines he gets (like Ranger Rick, but focused on science... the name escapes me). That article on the blood & circulatory system was the trigger for 6 - 9 months of intense focus on anatomy. I'll post some of the resources that you may be interested in. We were lucky enough to have the BodyWorlds exhibit near us at the time (www.bodyworlds.com).

When it comes to the reproduction system, we treat it as any other organ such as the kidneys, lungs, etc. (DS is a very good reader.) We had a discussion--reviewed a couple of times--talking about what is appropriate to talk about within the home setting and what is not appropriate to talk about with friends. Somehow we found a good balance with that without making it into a big stigma. But other than that, we don't have a problem with our children reading material that is "age appropriate" about sex. He understands a lot of the process and biology of reproduction, but not necessarily the sexuality part of it. At BodyWorlds, all of the bodies are naked (and the skin is peeled away) and one of the things that he was naturally curious about was "daddy this is a male... daddy this is a female." We're probably a bit more open than most people. But he'd also point out the kidneys, bladder, digestive system, etc.

So of the books that we made available to both DS5 and DS2 (at that time) were books like:

Where Willy Went (still a favorite)
http://www.amazon.com/Where-Willy-Went-Nicholas-Allan/dp/0375830308

It's not the Stork
http://www.amazon.com/Its-Not-Stork-Families-Friends/dp/0763633313/ref=pd_sim_b_4

It's so Amazing
http://www.amazon.com/gp/product/07...IKX0DER&pf_rd_r=1PDAB7T230YMQR1E10G0

(We were also pregnant during this time.)

While I am sure that we warp our kids in others ways smile this approach has worked out very well for us. Like I said we're pretty open about it and treat the subject matter almost like a clinician.

Some online resources we've found and enjoyed:
http://www.visiblebody.com/ (the best and IIRC you can turn off sex organs)
http://www.medtropolis.com/vbody.aspx
http://www.innerbody.com/htm/body.html

Hope that helps and is not TMI...

JB