Cut two identical pieces from your plank using your jigsaw that are each 10 inches long by 8 inches wide to make the sides and top of your footstool. Use a right angle to make sure your cuts are a perfect 90 degrees. Mark the 8-inch side as the top. Decide which piece will be the right side and which will be the left side of your stool.
Make the top piece of your footstool by cutting a third piece from your plank that measures 9 inches wide by 10 inches long. Set the top aside.
Attach your side pieces to the cross brace using two screws on the outside of your side piece, going into your cross brace. End with an "H" pattern with the 1-inch side being the width, the 10-inch side being the length and the 2-inch side where the screws attach from the outside of the stool to make your base.
Place your stool right side up and center your top step on the base. Measure to make sure that there is a 1/2-inch overhang on the sides of your stool. Drill two screws into the sides of your stool from the top in a 90 degree angle on the left. Repeat the procedure by screwing the top step into the right side of the base through the top.
Router or sand the edges of your top step and add a decorative design to the sides of your footstool that can be cut out using a jigsaw to personalize your work. Sand your footstool to remove any debris or splinters.
Finish your footstool with a fresh coat of paint in kid-friendly colors or paint their name on the top and use a clear lacquer to set it. Allow to dry for at least 24 hours before using.