As the mom of girls, I explained simply about it & let them know it was no big deal, a part of life--like brushing your teeth & going to the bathroom. We were always together, I never hid it, but I did try for some privacy, not that I got it too often, but...so it just seemed like one day they noticed the pad & blood in my underwear & asked. I remember my youngest running around the house yelling "Mama's got BLOOD"!!...& of course, the next time they saw I was on my period, they were like, didn't you already do that? So then I explained that it would happen each month, if I didn't have a baby in my ponca (don't know the spelling). It really did take each girl a couple of months of noticing when I was on my period that it was going to happen again.
As they have gotten older, it has dawned on them that as girls they would be doing this themselves, & I have gotten more questions, but I just answer them as honestly as possible. They were pretty horrified that a) it was going to happen to them, like it or not & b) it kept happening--each month!!
I also bought that American Girl book "All about Me" & it is just around for them to check out, if they want to. We talk, but this is one subject that evolves over time & there are always things you don't think of that come up that need a little extra explanation.
One of my girls (she was 2 or 3 at the time) thought it was really neat to watch the tampons expand in the toilet...got 1/2 a box in the toilet before I caught her. Another of my girls took to wearing a pad in her underwear everyday for a couple of months after the AGE class in 4th grade...she was 8 or 9 at the time. I explained the waste of that, & since I didn't start my period until I was 12, she probably had some time yet. But she was really worried for a while that she would have an accident at school & everyone would make fun of her.
As the mother of boys, it may seem like you don't have to to explain it to them, but please do!! They are going to grow up & hopefully be husbands & fathers & the more they know, the better husbands & fathers they will be. There are simple books in the library you can check out when they start asking more detailed questions to help you explain.
BTW, one of my girls DID tell her friends at school that I bleed from my vagina...what can you do, but laugh? And then explain that most people don't want to know that!! Good Luck!