I used an old lambswool jumper of Craig's that had holey elbows. You can make at least two children's cowls out of one jumper, but might be able to squeeze two more small ones depending on the size of your jumper and its neckline (a V-neck will of course give you less fabric). I haven't tried it yet, but I think you could also make a nice wide adult's cowl by using the whole bottom half of the jumper. For a child's cowl, cut open the side seams 25cm from the bottom edge and cut across the jumper like in the illustration.
Use a zig-zag stitch or overlock the raw top edge of the rectangle. Place this edge on the original bottom edge of the jumper, right sides together and stitch across. Turn the tube inside out and position the new seam at the bottom of your cowl.
Finally place right sides together (if there is a difference) and stitch across the two raw edges. It would be a good idea to try it on for size at this point, so you can make it a bit smaller if necessary. Finish the raw edges with a zig-zag stitch or overlock. Turn the cowl inside out and hide the exposed seam in the back of the neck.
There are probably other ways of making it look nicer, but I think the speed and ease makes it worth trying. Have a lovely Friday!