My DD is 11 and wears clothes ranging in size from size 8-10 (some tops a 12 so they're long enough to cover her tush). For pants she usually prefers jeggings from Target or Justice. She's gotten away from the "cutesy" at Justice, but this year their stuff does seem a bit better. She's gotten a couple of drapey cardigans, etc, from there, and she actually wears them.
She loves anything Under Armour, Nike, and many things from abercrombie kids. These days she most often lives in hoodies and jeggings/leggings. Occasionally, she'll wear sweatpants, if they're from one of the above brands. A drawstring is a must as her waist is so tiny, and those brands have them. Yoga pants typically have too wide of a waist for her, she hates the fit of "regular" jeans, and chinos/khakis? No way! She just can't stand those elastic/button adjustable waist combos, skinny fits are too tight in her thighs, and regular fit is way too wide at her waist.
And a quick note: she can't wear abercrombie kids jeans as the thighs are far too tight. Same goes for Ruum, but others have had luck there. Also, Forever 21 has a kids line online (have bought DD leggings from there... they're thin), and J. Crew kids is generally pretty cute but some seasons is hit or miss.
Also wherever you can find Guess? That tends to run smallish, and isn't too cutesy. Same with Ralph Lauren at Macy's/Boston Store/etc and their Chaps line at Kohl's.