The Squirrel and I set out to make a stencil. I grabbed a file folder because of its thickness. She drew a cat on it and I cut it out.
The file folder worked out great for a stencil because of the way you can set the paper inside it-- the bottom edge helps with aligning and keeping it in one place.
And when we decided to do a second color (the cat's stripes), I could easily trace the shape onto the back of the folder when closed, then open it up and cut out the stripes...
...and if she aligned with the folder's bottom edge when painting the second color, it turned out pretty good.