The axle and wheel, a wedge, an inclined plane, a lever and a pulley, as well as a screw, are six simple machines that kids can make for a physics lesson. The machines, which can be constucted from simple household items, demonstrate all of the founding principles of physics.
Make a basic wheel and axle machine by putting a pencil through the center of a spool of thread. Roll the spool of thread back and forth to demonstrate how it reduces friction.
Use the same pencil and thread spool construction but hold it up in the air and drape a string over it to create a pulley. Tie an object to one end of the string, and use the other end to pull it up and let it down. This demonstrates that a pulley doesn't change the weight of an object, but it does change the direction of the force needed to move it.
Use a ruler placed centered over an object such as a small block of wood or an eraser to create a lever. This demonstrates the power of mechanical advantage. Slide the ruler so that one end is on the block to create an inclined plane. This shows how objects can be elevated by moving up a slope.
A screw can be made by wrapping a piece of paper around a pencil to create an inclined plane that is affixed to a pole. A wedge can be a simple triangle-shaped wood block pressed against the pages of a closed book to force them open.