We used a computer/printer cart to make our workbench. You could also use a nightstand, side table or cut down a shelving unit. The key is to make sure the piece is adequate height for your child.
If there is an open back to your piece of furniture, measure the opening and cut piece of plywood to cover it. We made our plywood backing about three inches taller than the table. The added inches will sit above the new work surface and be used to support the vertical pegboard. Paint the backing and let it dry.
If adding a new top, measure then cut the plywood so there is a small overhang in the front and on one side. We made a bigger overhang on one side to allow for holes in which to hold tools. Attach the new work surface top with wood screws.
Attach the backing to the back of the unit using wood screws. Remember, the backing should sit a few inches above the top of the work surface. Hold the pegboard up against those few inches and attach the pegboard to the backing and to the work surface with L-brackets.