I saw instructions once for the cape. You cut three panels the length that you wanted, sewed them together and hemed the bottom. At the top you made a header (like on a curtain) with a casing and ran a cord to gather it up. I did that 3-4 yrs ago out of some netting with flocked pumpkins. It turned out cute. I would think for the dress that you could take twice her length (if it's 45" wide), fold it so you had a length in front, a length in back, cut a hole for her head. Then put it on her, measure down for her arms and place some pins so it would give you an idea of where to make your side seams. She could wear a black leotard under it. You could then cut the skirt in jagged points and leave it frayed and she could belt it with the same cording for the cape.
Just my 2 cents. Good Luck,
Susan

Many thanks Susan...you've been a great help! I've just finished the cape (to your instructions) and it's turned out great! The dress will be done tomorrow so all in all it will have only taken me around an hour to make a complete outfit! (shes already bought the hat and broomstick) I'm so glad I posted on here for help! Thanks again!
Karen UK

Karen,
You're welcome!!! The year I did my cape I bought a cheap witches hat and hot glued the black boa/feathers to the edge. It made it look "high end" for just a few dollars. The hats with the feathers are expensive here. You'll have to post a pic for us to see.
Susan

I'll do my best Susan! I'll get her to take a photo and then will have to get my dd to show me how to put it on here...(can you tell I haven't had a computer very long) lol! Still, I'm learning something new every day!
Karen

another way to do a cape, w/o gathering the neck, is to make a pencil-and-string compass, and draw semicircles. This provides a very smooth neck and chest area for the cape.

You can also, for a fitted cape, lay two slim rectangles (the front pieces) on a wider rectangle (the back) and sew up the edges, and curve in for the shoulders. Then you can cut a shallow semicircle in the center for the neck itself.

Here is a link that might be useful: semicircle kids' capes from Martha Stewart

Thankyou Susan and Talley Sue! Just thought I'd let you know that the outfit turned out great! Just as soon as she brings me the photo I'll let you see how it looks....(I think I know now how to put a pic on here!) All the best to you ....Karen