Could it be possible--just maybe--that little girls as young as 6 may perceive themselves as "sexy" because they are sufficiently old enough to be aware that the female form is naturally beautiful? Granted, our society puts a combination of positive and negative factors on physical beauty and the female expression of it, but outside of all the culturally created moralism surrounding these attitudes (note value-laden terms like variants of "objectification"), the bottom line remains that it's simply natural for males to find the bodies of females of disparate age groups to be beautifully attractive (i.e., "sexy"), and for girls to pick up on this fact, because this is how nature intended it. The cultural attitudes and mores surrounding this natural phenomenon is simply yet another attempt by human culture and moral precepts (specifically, Victorian-based precepts) to attempt to regulate nature and to control human responses to it. The lack of our ability to subvert and sublimate important aspects of nature that go against certain doctrines of moralism are posited as a "problem" by our society, and this is a good example.
I just finished the newest essay I promised a few weeks ago (it will be appearing in the journal published by the crew on VoA soon), and I am now going to start outlining an essay for the phenomenon of "self-sexualization of young girls" discussed in the article Shaper linked to, and the article from Life Science that it in turn links to.