Select two identical bookcases to use as the supports for your desk and measure their width. If you can find two bookcases that are the desired height of your finished desk you can turn them sideways to support the desk top while also providing storage space. If you cannot find bookcases of the right height, remove the top of the bookcases and as many of the upper shelves as necessary and cut the bookcase to size.
Use a tape measure to map out the height, width and depth you want your kid's desk to be. For a small room, build something long and narrow to utilize the space effectively. Larger rooms can accommodate wider desk tops. The desk top should be equal in depth to the width of the bookcases
Cut a sheet of 1-inch plywood with a table saw to the dimensions you determined for the desk top. Sand the edges and surface of the plywood with fine-grain sandpaper to remove imperfections. Fill any depressions with wood filler and, once it dries completely, sand it smooth.
Lay the piece of plywood flat on the ground upside down and position one bookshelf upside down on each side, aligning the edges.
Measure the total thickness of the plywood and the top of the bookcase to determine what size screw to use to connect them. Use screws about 1/2-inch shorter than the thickness you measured.
Pre-drill holes for your screws with an electric hand drill - the hole should pass all the way through the bookcase top and extend just into the plywood. Position one hole in each corner of the bookcase top, one inch from each adjacent edge. Drill additional holes every 4 inches along the long edges, measuring from the hole at the corner.
Secure the bookcases to the plywood desk top by inserting wood screws through the bookcase top into the plywood in each of the predrilled holes.
Flip the desk upright and place it in the desired location in your kid's room. If you like, you may paint or stain the desk top to match the bookcases or the decor scheme in your child's room.
Fill the bookcases with your kids' books, toys and other items. Use the flat surface of the desk top for pencil holders, notebooks and craft supplies.