I don't believe that kids need to wear name brand clothing. They're still children who play games and get their clothes dirty. It would be a waste to spend $100 on a pair of jeans and have mud stains all over it. Plus, kids grow out of their clothes pretty fast, so I wouldn't recommend spending lots of money just so they have the word "abercrombie & fitch" on their chest.
Tween fashion can be difficult. Gymboree is too young, Forever 21 is too old. Try Ambercrombie Kids. The styles are modest, sporty and on top of the trends! They are basic without being boring!
Glendale Galleria offers a storefront so you can let your tween shop to his/her heart's content. The mall features other tween friendly shops like GapKids and Justice.