The marshmallow mixture is sticky. Letting it stand for a few minutes decreases the stickiness. It is also helpful to spray a rubber scraper with cooking spray for mixing the marshmallows into the cereal. Also, spray your hands very well to press the mixture into the cookie cutters.
Chop the cookies by hand as you would onions, or crumble them and pulse in a food processor until they are about 1/4-inch pieces. Do not overprocess.