bulldozer and brooklyn both gave perfect answers...trying to make her stop, will make her think it's "ugly" or wrong...and in no means is it either one....as bulldozer said, tell them not to do that in public....baby boys play with their "pee-pees" and baby girls will try to put stuff in there, when they figure that there's a "hole" down there....then again, could it be she does it b/c she is itching, therefore it feels good....?
rubbing on a hard corner, is something *I* would do if i was itching, not because it feels good....
With regard to any kind of masterbation in children, yes it is normal but they should also be made aware that it is not something they do in public. A straightforward, "if you want to do that can you do it somewhere private", then turn away. It worked with my children.
You will not get her to stop! It is normal for children that age and any age to find out that rubbing there selfs feels good. They dont know why that is but it does. My daughter has done it for years since she was about 5 the first time I court her, I did not say anything or even look twice and you should just to no notice of her when she does it as its part of there growning up and finding out about there body.