''The Invisible Boy'' is a mixture of lighthearted playfulness and menacing evil. In 1957, ten-year-old Timmie Merinoe (Richard Eyer) seems only to want a playmate. After he is mysteriously invested with superior intelligence, he reassembles a robot that his father and other scientists had been ready to discard as unrepairable junk. No one pays much attention to the robot, named Robby, after Timmie gets it operating again, until Timmie's mother becomes angry when her son is taken aloft by a huge powered kite that Robby has built at Timmie's urging.