Having made lots of costumes, I wouldn't bother to interface either. But if you really want to, another layer of white cotton or use a light weight fusible interfacing. Follow the direction that come with the interfacing so it doesn't shrink when you fuse it. Especially for a child, I wouldn't bother though. Not sure how much the light weight is going to cover. Really don't think anyone is going to notice. Have fun and lets see some finished pics.