OP, I would have done exactly as you did. I have one son (he's 10) but lots of nieces. Well, as the nieces have gotten older it's been a tradition that I take them shopping and we go out for a "girls" lunch. Two of my nieces are pre-teen/teens and I inform them BEFORE we go, that no "short shorts" are allowed, no shorts with anything written across the tush are allowed, no shirts with suggestive messages are allowed and they cannot show cleavage!
One of my nieces recently object to my policies and I simply informed her that I was sorry she wouldn't be shopping with me, that we would miss her and perhaps she could come next year. Needless to say, she changed her tune very quickly!
I will add that I give each of the nieces $100 to shop with...but that I have veto power on what they buy! We talk about the cut of things, how things drape or stretch and why that is good or bad, fabric choices, fabric care, etc. We also go out to a nice "ladies luncheon" type of place where the girls are exposed (ah the horror of it) more than 1 fork....more than 1 knife...and more than one spoon!
Am I "old-fashioned"...yep, I guess so. For that I thank my great-grandmother who insisted that I learn how to properly set a table, curtsy (yes, that was over the top) and learn how to select appropriate clothing.
I don't think there is a thing wrong with insisting that a pre-teen/teen dress conservatively. I think parents actually do a disservice to their children when they don't teach them proper dress and the like.
Do I consider myself "stuffy"? No, not at all! In fact, I'm the "fun" Aunt who takes her nieces and nephews on trips (including Disney)! I just think that we owe it to our children to teach them what is proper...and lord knows some of the clothing today (and when I was a kid as well) leaves NOTHING to the imagination!
I will also add, that I refuse to buy shirts from Hollister, Abercrombie and the like as I do not see the need for my nieces to have their breasts labled with someone's name! When I told the nieces that, they laughed out loud, said they'd never thought of it that way and also agree that it's stupid to have someone label your breasts!