Cut 3/4-inch plywood into the following sizes: four 36-by-16-inch pieces for the front and back, two 16-by-16-inch pieces for the sides, and two 36-by-17 1/2 pieces for the top and bottom.
Assemble the front, back and side pieces into a box. Drill 1/16-inch pilot holes every 4 inches through the corners, then assemble the box with a 2-inch wood screw in each pilot hole.
Countersink the screw heads slightly below the wood surface.
Place the bottom piece onto the box. Drill pilot holes through the bottom and into the box every 4 inches and secure the bottom to the box with wood screws.
Flip the box over so that the open end faces up.
Cut a 36-inch piece of 2-by-12 board, and two 15 1/2-inch pieces of 2-by-6 board.
Round one corner of each 2-by-6 board with a jigsaw, and two corners along a long side of the 2-by-12 board.
Assemble the three boards onto the top piece of plywood, forming the back and arms of the bench. The rounded sides should face up, with the rounded corner of each 2-by-6 facing forward.
Drill pilot holes through the back of the 2-by-12 and into the 2-by-6 boards. Use 3-inch wood screws to permanently connect the lumber.
Drill pilot holes through the underside of the top plywood into the lumber and connect them with 2-inch wood screws.
Place the top onto the toy box. Line up a 36-inch piano bench hinge along the rear joint between the top and box.
Screw the hinge in place with the attached screws.
Fill the screw head holes with wood putty, using a putty knife to scrape the putty smooth. Allow the putty to dry.
Sand the entire toy box with 120-grit sandpaper to smooth the surface and remove any splinters.
Place cushions on the toy box top if desired.