Two other recommendations on this general category:
1. I'd encourage ANY parents with anxiety about this issue to read Debra Haffner's wonderful From Diapers to Dating: A Parent's Guide to Raising Sexually Healthy Children. It offers SUCH a good approach....instead of one Big Talk, she promotes a much more casual, ongoing dialogue, just the same way we return time & again to other important life topics - kindness, honesty, household safety, eating & sleeping, whatever. (I believe that Haffner also recommends the "What's Happening to My Body" series, by the way.) I had trouble being casual about this stuff at first, but I've realized that it's SO much better to just treat it like any other topic, while they're young. If they don't see us acting freaked out about these things, they will be so much more likely to 1) not think of their bodies as dirty/taboo, 2) come to us for information instead of picking it up on the playground, 3) learn to take responsibility for themselves and their decisions as they mature into adulthood, rather than just getting "swept away" by hormones & ending up with unintended heartbreak/pregnancy/disease that could SO easily have been prevented.
2. For parents w/ younger grade schoolers, check out this book by Marc Brown (yes, from the Arthur series) and his wife (Lauren Brown, maybe??), a social worker or child psychologist, called What's The Big Secret: Talking About Sex With Boys and Girls. The illustration style offers comforting familiarity to any kids who've read or watched Arthur, and the information is presented in a basic, matter-of-fact way.