When my kids were young and did Halloween (actually they did Halloween until they were about 17) we never bought costumes. We always made them. The kids were usually pretty good about coming up with ideas. I'd buy accessories like a hat, or wings. One year my Dd went as a fairy. I bought a pair of wings at Wal-mart and she already had pink tights and a skirt from ballet class although those things can be made and/or bought cheaply. One year Ds wanted to be a robot. I took a cardboard box and covered it in aluminum foil, bought some silver duct work for the arms, added a calculator, some bicycle lights etc on the front of the box, used some silver face paint and he wore gray sweat pants. For the feet I took shoe boxes and cut out a hole for him to slip his foot through and covered them in aluminum foil. For a fairy costume you can make a long skirt out of an adult skirt. Just take elastic and make the skirt tighter or if she wants a flow y gown take some silk cloth and sew it down to sides, then put elastic around the top. The elastic would go around the neck and you would cut out holes for the arms, then add some wings from any costume area and some glitter. You could even add a cheap tiara from a party store or even a dollar store. The possibilities are endless if you think out of the box a little