Well, in truth, your kid would be in line for a Nobel Prize if s/he invented a simple machine! Perhaps you really mean "invent a new compound machine"?
Six simple machines have been identified - Lever, Inclined Plane, Wheel and Axle, Screw, Wedge, and Pulley, and they make work easier for us by allowing us to push or pull over increased distances. Compound machines are combinations of these and most machines are compound machines. Compound machines can do more difficult jobs than simple machines alone. Their mechanical advantage is far greater, too. Some examples are a pair of scissors and a bicycle. What I suggest your 8th grader do is to invent a "Rube Goldberg" device, combining a number of simple machines.