I have always taken my kids with me when I shop for them. I like to have them try everything on so we can be sure it fits, they like it, and it's comfortable. Actually, at my favorite place to shop, the sales lady in the children's department has been helping us for years. I call ahead, tell her the sizes, my budget, and what I'm looking for (dresses, jeans, t-shirts, whatever) so when we get there, she has some things pulled aside for us. That way, I don't have to wander around trying to figure out what outfits go together, my kids pulling out stuff that costs a fortune or that they don't need, etc. Then, I send the kids into the dressing room by themselves. If there's something they like, they come out and show me. If not, they put it in the "don't like" pile and move on.
This way, they end up with outfits they picked, and because I didn't have any part in choosing it, they like it even better. ;)
You know, I don't see why your kids would think Once Upon a Child was a bad place to shop. My kids wear hand-me-downs all the time and are fine with it. In fact, they like many of their hand-me-downs better than they like their clothes from the more expensive department stores.