LOL... If you're sending them to school... avoid the word "sex"... unless you want to /feel comfortable having discussions with other parents and teachers. Mating and procreating, however, seems to be entirely acceptable synonyms.
I went with the scientific explanation, and skipped the mechanics, when my son was 6ish. In perhaps a 5 minute conversation complete with DNA helixes. And we've seen some cool programs on Discovery & Nat.Geo.Soc. where they've shown eggs being fertilized. Didn't realize how much of it stuck until a year later when my mum for some reason asked my son if he knew what sex was (I don't exactly remember why... but I was right there, it was an innocent query).
He said
"Sure! It's how you blend genetics. You can't see genes blend though without a microscope. We don't have one, but you can watch it on TV. Watching the cells split is like watching a bubble bath. Lots of things can go wrong, but if it doesn't you get a baby SOMETHING. Even a baby alligator! Can you imagine if mom had a baby alligator? She HATES alligators. BIG problem for mom."
Okay, and apparently, we need to have the discussion again. But since he hasn't asked, I'm letting that sleeping dog (or alligator) lie until he does again or puberty, whichever comes first.