Chihiro from Spirited Away (either her bathhouse outfit
, or her street clothes
. And I know her hair's brown, but that really doesn't matter)
from Omashi Magical Theater: Risky Safety
from Chobits (your sisters hair is a little short for her, but since she's so young, she would make a great Kotoko, and again, that sort of thing is overlooked for kid cosplayers)
from Carcaptor Sakura/Cardcaptors (you could get some clip in extensions to slip under her buns, and lengthen the skirt)
Also, if you don't mind doing a character that's not a child, Sailor Mars
from Sailor Moon would be super, super cute!