Bake and cool cake. With soap and water, wash toys being used on the cake. With 2/3 of the frosting, spread over cake evenly (if you want chocolate, add 3 TBSP of cocoa powder for a light chocolate frosting). Leave a space in center of cake that will host the # of the age of the recipient or for their name or a message. Around the perimeter of the cake, spread crushed Nilla wafers on one half. This is the area you will put the combine and tractor/wagon. Sweep fingers over the Nilla wafers so there are mock 'rows'. Place combine and tractor on crushed wafers. Lay out crushed Oreos on the opposite side of perimeter of cake. Place CAT or other Earth mover in that area, scoop a bunch of the crushed Oreos into the blade or bucket of the toy. With remaining 1/3 of frosting, tint all but 2 TBSP green. Pipe inner area decoratively and write message or # of the birthday. Sprinkle with yellow sprinkles. Dye remaining 2 TBSP of frosting yellow, place frosting in hopper of combine and in wagon of tractor. Cover with yellow sprinkles to resemble corn.