The trick with the birds and bees talk, is not really that it's a "talk", it's that it will need to be an open, on-going dialogue between you and your children.
They need to feel they can trust to talk to you about these things, without embarrassment. So, if you're embarrassed, then they'll be embarrassed.
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I lucked out with my five year old. I got pregnant, he asked where babies came from, I told him a nice little fairy-tale. He looked at me, looked at the poster on the doctor's wall, and pretty much asked me again. He knew that wasn't the truth, we'd already gone over anatomy using the correct terms with him. So I told him flat out- "Mom and Dad had sex."
His response? "Oh, ok." Then he nodded. I didn't give him any further explanation- no "this goes into that and this comes out..." For his age, that's all the info he needed.
So, the first thing to figure out, is exactly how much information he needs. If he's not ready for instructions on condom use- don't give it.
Secondly, since he knows the difference between boys and girls, go ahead and teach him basic anatomy. Not the function or use of it, just the correct names. I personally would rather have my son ask if the baby is coming out of my vagina than any of the "slang" words for it that he might have heard at school by that age.
And last, let him ask a few questions. He might not know WHAT he wants to ask, but don't answer more than he asks yet. He doesn't need a FULL biology lesson at his age, but a good idea of anatomy, where/how babies are made, and the knowledge that he can talk to you in the future if he has more questions will go a long way to prevent future mis-education from his peers.