My girls have known for years what happens every month to Mommy and someday to them. They're 10 and 8 (and seriously, the way my 10yr old is developing, I wouldn't be surprised if it started happening to HER pretty soon, too). I've explained what it is, why we get it, how it feels, all in a very basic way without getting too technical. They basically know that women have a sack inside them where the babies can grow called a uterus, and once a month that sack cleans itself out, it's totally natural, all women go through it and it's mostly no big deal. They also know that sometimes when the body is cleansing itself, Mommy can be in a kinda bad mood while it happens and sometimes she can get stomach aches or cramps, adding to her bad mood. But mostly, it's no big deal and when it happens to them, they'll be totally ready. I remember getting mine at 11 (on an overnight school trip, no less) and knowing absolutely NOTHING about it and being terrified. I don't want my girls to ever go through THAT like I had to. So we're very open about woman issues in our house.