A few weeks ago, they had a puberty talk in school. When I asked Roo about it, I was surprised that it was mostly a nuts and bolts overview of male and female anatomy, not as much about what all those parts do when combined. The thing that stood out most to her was apparently a demonstration of how tampons work. I asked if any of her friends had started their period yet and she said two had. It was obviously something the girls have been discussing.
This video from Dear Kate shows women talking about when they first got their period. It's another good overview kind of thing to have in your parent-of-a-tween-girl arsenal. I think one woman might swear, though, so if you're offended by that you might want to steer clear. Honestly, my period still makes me want to curse like a sailor and I've had it for more than three decades now.
I don't know if it's just being prepared beforehand but Roo is less concerned about the whole concept of menstruation than I am. It's kind of a bigger deal to me than it is for her, which I am hoping will translate into an easier understanding of body and sexual issues as she grows up.