What is a good idea for a kids business?

Radio controlled warships. Hulls can be made of plastic, fiberglass or wood. Minimum length of the vessel should be no less than 24" (2 feet) so your hands can fix/repair/adjust the electric motors/batteries/wires/ballast inside the hull. Radio controlled warships are safe, they can't injure anyone, because they don't move fast enough & electric motors don't make noise.