What should a 12-13 yearold wear to her friend's batmitzvah?

My cousin is going to her friend's batmitzvah and she wanted me to post this. She's 12 gonna be 13 in a couple months. shes like 5' , weighs like 100 , size 0 pants and she needs a dress but we need help finding one. nothing from other countries like uk and stuff. Livin in united states. in virginia. and for the price- $30 at the most i guess. nothing too expensive. not too bright. ohh and shes only going to the party part of the batmitzvah. help