Good for you! Although I think from personal recollection that fourth or fifth grade would be more seasonable -- especially considering that gays are trying to get to kids to shape their values and make them receptive to homosexuality in elementary schools, as witness the appearance of Heather Has Two Mommies, etc.
At that age, your kids trust you a lot and if you say "there is a lot they are not telling you and I will tell you when you are a little older" coupled with "we don't mess in our poop because it makes us sick" it's enough for grade schoolers. : )