As usual, the onus of sexuality is on the female-- you don't see little boys running around in speedos.
It predates Miley, by years; she's very talented, but I assume her manager wanted he to break out of the Hanna Montana persona-- this started a while back, I believe with her "Lolita" shot on the cover of Vogue. Miley is a symptom of a problem. She's popular because she rides the edge between little girl and woman-- teddy bears, tongue, and twerking. A perfect storm of mixed signals, appealing to young boys, older men and quite frankly pedophiles.
She's very young, and I saw her on some show mocking "what about the children" --she's kind of an asshole really, but maybe she'll grow out of it.
As far as parents picking that kind of attire for their little girls-- I've seen it and it's bizarre. Girl children are not taught that they are sexual beings-- they are taught that they are sexual commodities no matter how cute mom thought the outfit was. They are aware of the eyes on them; the parts of the body those outfits emphasize even if most of them don't understand it. The blatant marketing of the female sex had grown, but there is an incredible amount of pushback on this.
I work with women mostly as a nurse, and I can tell you anecdotally this is on their minds, and supporting this over sexualization of their little girls is NOT what they're going to do. Nor are fathers for that matter.
It's a matter for worry, but it's very much something feminism is fighting against, even companies are fighting against it, because voices have been heard. Longways to go though.