How do you fit into the kids section with being 120lbs and 5'3"? Correct me if I'm wrong, but that's on the larger end of being average. Although you're short, I think her issue is more of being very thin as well.  I would recommend American Apparel. They run down to XXS. I'm on the taller end, but I'm very thin and it fits me fine. I would also recommend Zara since they do European sizes. Any Asian brand like Uniqlo also runs pretty small. If you don't mind spending extra, a lot of high end brands carry very small sizes.