What size to make? (the tween dilemna)

We are "teaching" a sewing class. We say teaching very loosely since Liesl will really be doing the teaching :). Two of the girls have selected the Gallery Tunic / Dress to sew and are both measuring size 0, however their hips are only 31" and 33". Should we make adjustments and if so, what would be the best method to accomplish this? Also, we possibly will need to shorten one and lengthen the other.