I don't know how helpful this will be...
I am somewhat addicted to watching "A Baby Story" on TLC. Both of my boys have seen parts of it enough, that they are getting the concept of childbirth. Pretty much they are horrified. Which frankly right now is okay.
There was also some special on Discovery Channel (? maybe) that was really cool where they did the whole fetal development using some fancy camera, it was probably 2 hours long. It was purely scientific stuff, but my son was so into it, we let him stay up late.
Neither of them deals with the menstrual cycle, and certainly not a substitute for parental discussion. But even I thought the Discovery Channel show was pretty neat.
My oldest accidentally walked in on me in the bathroom while I was dealing with pads, I tried to explain, his first thought was that I was hurt, something wrong with me. I would think from a kid's perspective, that would be a pretty normal reaction. What he knows, and he's a boy so I'm not having to explain so much how to deal with periods, more just explaining what they are... is just that it's a normal thing that happens when girls grow up. I tried to go into more detail about, you know, the egg blah blah blah, but I could tell he was zoning out on me so I shut up.
I think if I were in your shoes, for now I would let them know that if they do start their period, what it looks like and that it's okay and to tell a grown-up and that it might make them feel grumpy or crampy and they can talk to you and you'll help. Girls are starting earlier and earlier, and if yours are physically developing, it might happen in 4th or 5th grade. And I would give them the spiel about not letting anyone touch them etc. And then I'd wait a while before delving further.
I wonder if they still make those little "kits". I remember I got one of those "back in the day".

You might ask the school nurse if she has any booklets or anything, you might feel more comfortable if it's something scripted.