Little kids are not asking about the vagina when they ask what girls have. They want to know what the stuff they can see is called. And that is called the vulva. When a little girl only knows the word vagina, that is the word she will always use. For instance, I've heard this: "OUCH! He just kicked me in the vagina!" It would be very very difficult to be kicked in the vagina. She was complaining about being kicked in the vulva. The way I see it, the only way to be kicked in the vagina is to give birth to a very active baby.
And when a little girl grows up only knowing to call it all her vagina, that is what she will continue to say. I once heard an adult woman say, "The crotch on this underwear is too small. I can see my vagina." You'd have to be doing some serious up close inspection of your underwear to see your vagina and even then you really can't see your own vagina unless you're a stellar acrobat doing a little digging or holding a mirror and using gynecological instruments. What she was seeing was her vulva peeking out the sides of her underwear.
For people who may have no idea what I am talking about (...Gay Travis...) I will briefly explain a couple of things: the vagina is the tunnel that connects the outside to the inside, the uterus is the inside where babies grow and the vulva is the outside - there are lots of smaller components to it but I will allow you to do your own research for details. I am not, however, going to suggest any google search terms because I do not want to be responsible for anyone arriving at a porn site.
By the way, I once discussed this with a medical professional/feminist friend and we decided that the problem may be that our society needs a male version of the word. I hereby officially rename the penis the vulvo.