since she is already 6 i would suggest you talk to her about it, it is normal because she is exploring her body and its completely natural, but she is old enough to understand that it is not something that she should be doing in public (not that she is but just to make sure she is aware) and if you are concerned you can always talk to her doctor about it it can't hurt...
I agree some is normal in children but it is generally agreed excessive can be worrisome and a sign of abuse, I don't think its real masturbation at 2 year old .Certainly most children seem to do it ..its probably a comfort thing ..
This is very normal. Most kids do it, because it feels good to them. They do not understand why it feels good they just know it does. My sister has written a journal about it. You can find it here
http://www.medhelp.org/user_journals/show/320368/PSA-About-Little-Kids?personal_page_id=669869