Set the two 25-inch boards on edge and 13 inches apart. Set the 13-inch boards between them so they're perpendicular and their sides are flush with the ends of the 25-inch boards. Screw four screws through each 25-inch board so two screws enter each end of the 13-inch boards. This is the frame for your toddler workbench.
Position a 15-inch board inside each corner of the frame so all four are perpendicular to the frame and their ends are all flush with one side of the frame. Screw through the frame and into the boards, using two screws for each corner. These are the legs to your toddler workbench.
Turn the frame upright so it's standing on the ends of its legs. Lay the plywood on top so its edges are flush with the sides of the frame. Screw the plywood to the frame using eight screws.
Sand the toddler workbench with the medium-grade sand paper, wipe it clean with the tack cloth and apply your wood stain. Sanding the workbench will prevent the child from getting splinters.