Cut away the two shorter sides of the box to a depth of 3 to 4 inches, with scissors or a box cutter. The sides left full height should be no more than 7 inches apart, so a pencil can span them. If you're using a wooden rod rather than a pencil, the width of the box can vary to match the length of the rod.
Make a matched pair of holes in the middle of the full-height sides of the box, using the tip of your knife or scissors. These will hold the pencil or wooden rod in place, making an axle and converting the spoon into a pulley.
Slide the pencil or wooden rod through the outer surface of one side of the box, about halfway through, then slide it through the empty spool and finally through the hole in the opposite side of the box. Tape over the holes on the outer sides of the box with duct tape, to keep the pencil from sliding out of its place.
Choose a small, heavy object that will fit inside the box, such as a coffee cup or a toy. Tie it to a 2 to 3 foot length of string and ask the kids to lift it. Then put the object inside the box and loop the string over the pulley. Have the kids lift your object by pulling down on the string, to learn that it's easier.