J.F.
I have 2 friends who's daughters did this and the grandmother decided that they were being molested (they were cousins) and insisted that we take them to the ped office and when we told him what she was doing he handed her a pamplet to give to her m-i-l and told us this was normal for some little girls. For those of us with boys they constantly have their hands in their pants tugging on their privates and in fact the other day when my 5 year old son was getting ready for school I noticed he adjusted himself just like a grown man and I have never seen his father do that in 27 years of marraige.
Out of the two little girls one has stopped humping toys, blankets and the floor but the other one still humps her stuffed animals. We told her that if she was doing to do this that is was something private and that she should do it in her bedroom with the door closed by herself. If you make her think it is dirty and something to be ashamed of she will harm her but it you treat it as something that is not shameful just private she will not think something is wrong with herself.