Talk to Them Early
Incredibly, your actions, words and presence usually don't humiliate your child during the pre-adolescent years. You haven't yet become the "worst thing ever," and you may even get away with grabbing a hand when you cross a busy street together without causing a full-scale meltdown. As such, these late preadolescent years are a fine time to talk about sex for the first time.
If you miss this window, it can still be done while they're growing through adolescence, but the awkwardness (and their periodic, intense dislike of you) will make it that much harder to bring up the subject of sex for the first time. Preadolescence is a special, cherished time when you can manipulate your children's thoughts, mold their impressions of the world and plant seeds of caution and restraint without them getting wise to your schemes, so take advantage of it.
Your preadolescent child will be receptive to what you're saying, and not embarrassed -- or at least not too much. Having laid the groundwork to a still-receptive mind, you'll be able to build upon it with greater ease once adolescence comes along and potentially swings a wrecking ball at your family's communication.
And you don't have to have just one big talk. Try periodically having small talks about sex. How periodically? Advance the conversation over time, but not so often your teen wants to melt into the ground and disappear every time you enter the room.
Next: The art of faking cool detachment.