If you're are not a seasoned seamstress don't stress yourself out by forcing yourself to sew. You can easily make cute costumes by cutting cheap articles of clothing you already have on hand or find at a store.
For example, Peter Pan and can be made from an oversized T-shirt and cutting some points out of the bottom and tying it with a sash. And the sash doesn't need to be sewn. A strip of fabric will do the job just fine. Also fabric glue will do the the trick in many instances.