I think too many girls these days have feminine confused with trashy, revealing, provocative, sleazy, and, I'm sorry to say, whorish.
There is nothing wrong with a 14 year old wearing a nice once piece, or even a 2 piece, provided all of her "girly" parts are covered. But in my opinion, to allow her to wear something like a thong or anything that shows parts of her body that nobody other than her husband has any business seeing teaches her nothing other than how to disrespect her body and how to allow men to view her as a sex object.