I used to be on the chubbier side too, was quite depressing not being able to get clothes like all my mates had.. I've lost the puppy fat now (i weigh 2.7 stone less than what i did when i was 13 !!) but its so hard for kids at that age.. its so sad how weight is focussed on at that age.. really saddens me!!
BUT.. if you shop at H&M you can get some GORGEOUS clothes for a cheap price.. the quality is good aswell.. If you buy her stuff from the adult section.. there is sooo much to choose from that she will be able to wear the same styles as her mates who shop in the kids sections because you can find anything if you look hard enough at H&M!!!
PLUS.. if you buy her something from the adults section.. you can get an XS size which is what I wore at about 12.. its the quivilent to a size 8-10.. im sure when she sees the "extra small" label she'll feel really good about herself... plus theres no difference between the adults and kids labels so noone will be the wiser!!!
(its best to go earlier in the day when its not full of bargain mad students .. quieter and neater!!!
good luck.. DEFO try H&M though... i get like, all of my clothes in there!!
oh.. p.s) they do some BRILLIANT accessories and things aswell.. nothing really costs above �4 (jewellery or make-up wise) so you'll be able to get her a nice little goodie bag full or wearable treats!!!