Store bought wings are so cheap and so perfect for what you want...I'd just buy the wings.
One year I even got wings with little faerie lights in them, they ran off a battery and twinkled.
I try to include some light of some sort in every Halloween costume. I like to be able to SEE her at all times. If I can't build lights in in some fashion, I buy bike lights and hook it on her collar or something. She hates it but knows she has to suck it up to be safe.
If buying wings won't work for you, find two pieces of fabric, pretty scarves or tie dye or pieces of sheet, cut them in a basic triangle, make the bottom of the triangle fancy, with trim or just cutting shapes out, and then safetly pin or sew them to your childs arms and body.
Or.....just put a sheet over her head, and cut thumb holes so she can flap.
I don't know where you are, but I make the basic body of the costume warm clothes. For a faerie, I might use green turtleneck and green leggings, etc. One year Maeve was Pooh Bear, she wore a yellow sleeper suit, a yellow hat with felt ears on, and a red vest, and it was adorable.
Warm and bright, that's the ticket.