We've been very upfront with our kids about biology and the reproduction of any species.... earlier with animals than humans, since we have two boys and waiting till when we did, we still got one kid looking like he was gonna hurl all over the place. But when they think of animals mating, it is all very natural and something they are fine with. When one was much younger, about 5, he asked if we could get a male gecko for our female so "Kala can lay babies"
I also was told as a small child that hens lay soft eggs so they don't break when they fall out of the trees, and they harden up later. I grew up thinking that farmers went on daily easter egg hunts, picking up eggs from fence posts, out of trees and stuff like that. My father was even in to 4H as a child, but I think it was mostly pork end of things.
So, I have talked it over with our 12 yr old son, and when we slaughter our first batch of meat birds, we will jar up and float the reproductive systems of a cock and pullet in formaldehyde for science class - we home school, so the chickens really work out well all the way around for us. Hopefully by Halloween, we will have assembled an entire chicken skeleton wired together and cleaned for keeps.