Girls' T-shirt dress

Both too wide AND too short. My daughter is 4'11" and 86 lbs. According to the sizing guide she would be an XL or 12-14 size, but she was swimming in this and every other dress we tried. However, when she raises her arms up the hem reveals her bottom. We love JCrew and I would buy the smaller size but it wouldn't solve the too short problem. Why design such revealing clothing for children? We have not had this problem with JCrew in the past. The fabric seems like it would be good quality, but we are returning.