When Charlie is on the swing, he is demonstrating all three of Newton's laws of motion.  Sometimes, Charlie will just sit on the swing and not actually swing back and forth.  In doing so, he is demonstrating Newton's first law of motion, which states, "Every object continues in a state of rest, or a state of motion in a straight line at a constant speed, unless it is compelled to change that state by forces exerted upon it."  Since Charlie is not moving, the swing is not moving.

Charlie would be able to demonstrate Newton's second law of motion by taking a step back to start the swing.  Newton's second law of motion states, "The acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object."  If Charlie was able to take several steps back before starting to swing, the force would be added proportionately to the swinging of the swing.