aksor, I'm sorry to hear your family has to go through this. My almost 11 year old is wearing a different brace, so I'm not sure if this would work; however, we have found knit "jeans" at Kohl's (made by Candies) and at Justice. They look like very stretchy five-pocket pants. Luckily, "sweatpants" have become fashionable, and you will also find stylish sweats and crop sweats at Justice as well.
We have purchased everything a size larger than what she would normally wear to cover the brace. Winter has been pretty easy--tunic length sweaters, loose fashion sweatshirts. Spring/summer will be a bit tougher, but I've noticed Justice is selling tunic length sleeveless tops, so we've got that going for us!
Here's a whole thread on this:
http://www.scoliosis.org/forum/showthread.php?t=8934