My daughter is 17 now, and thankfully we never had a problem finding tasteful clothes for her. She never wanted anything written across her butt, in fact, she has never even liked shorts or short skirts. Her friends have always dressed tastefully, too. However, going to the middle and high schools over the years has always been a real eye opener, looking at most other girls' clothing.
We've had a few Presidents in recent years with young daughters, and sleazy clothes have still been popular with young girls. I doubt Obama's daughters will much effect on the fashion industry, but we can always remain hopeful.