Why not knit a costume? I went looking and these patterns were available free of charge.
The Darling Devil Set available free from Lion Brand makes for a quick knit of horns and tail for any age.
If you're looking for a quick fix to not finding a cool costume and it's Halloween day itself. Take out your knitting needles and sink them into this pattern. Flying Spaghetti Monster According to the pattern, it only takes 4 hours to make this costume and gauge is not an issue! No swatching involved.
Now for something retro. How about a Hallowig? Knitty offers a cool pattern for a knitted wig, but better start it today or you may not have it done in time.
Another option offered by Knitty is the Jackyll & Hide hat. If you live where it gets cold by the end of October as in snow flying and wind blowing up your breeches, this would be a great mask. It does double duty as a hat and scares the living tar out of kids when you pull it down.
The book contains patterns for wizardly items such as: a wizard hat, robes, scarf, Errol the owl and more. If your little ones want to walk around looking like Harry or Hermione, this is the book for you.
For fans of Futurama, the Brain Slug from Mychawd, Knitting SAMurai 's blog will be a sure hit.
For home decorating or gift giving, this knitted pumpkin pattern is adorable! You can make it any size you want, just change needle size.