While it may be true that skimpy fashion has existed for young girls in the past, I would have to say that the fashion industry has taken it beyond what it used to be. The word pre-teen first started in the 60s...but it seems that the fashion industry and the media have hyperbolized that stage of life so much, that people in that age group begin to believe that they're adults already and they begin to want to dress and act like adults. Little kids are knowing about sex at younger and younger ages and the things they see marketed at them on TV (shows, products) and in stores have a lot to do with it. It's not even just the so-called "tween" clothing either -- they have clothes designed for babies that make them look like they're ready to hit the clubs after their next diaper change. Next, they'll think of baby-thongs...who knows? (And I'll bet people would actually buy that crap for their kids).
While nature will have an effect on how we and our kids feel (I'm talking about 'desires,' people), it does not mean that we should give in to everything we feel. (If I felt like raping, killing, or beating on someone, does it mean I have to do it? No. Absolutely not. This means I have to practice self-restraint.) Anyway, I completely agree with you and I do not believe you are wrong for feeling this way. I actually congratulate you on having such a wise daughter...unfortunately other people would think that it's "horrible" for a young girl to be so 'conservative.' (For reasons beyond my understanding).
Anyway, kudos!