I recently discovered American Eagle and we LOVE it! They stand behind their clothing, unlike some of the other retailers for that age range. I think it is a little grown-up looking for an eleven year old, at least the way dd dressed at that age - she was into Boden, Justice, various NR brands and even some Naartjie, and that sort of thing. At least go to AE for jeans for now and later on for other items. DD is 13 and it's just right for her now.
I wish I had known about OOO sizing before today! We were relying on slim cut skinny jeans from Old Navy and Gap, but the AE OO regular works for her now. She is over 5' 2" and probably 90 lbs at most.
We also go to Aeropostale, H and M, and the dreaded Forever 21. You have to be careful there, but we have found some good stuff, to my shock. I still prefer AE on sale. Don't count out Justice. We still get tankinis and BRAS from there - we really like their bras, of all things, and they have a good selection of small sizes. Women's sizes don't work for dd, and the styling wouldn't have worked at that age, though she likes a lot of the Inc. stuff I get from Macys!
We avoid A &F and Hollister for various reasons - cannot stand their philosophy - but they are very slim cut. Some of their XS clothes were too slim for MY dd, who at the time wore XXS everywhere, which says a lot. At the time I didn't realize they excluded girls with larger body types and wondered how on earth anyone but dd's type could fit into those clothes. ugh.
"Friendship is born at that moment when one person says to another, "What? You, too? I thought I was the only one." C.S. Lewis