That kid was Bill Mumy and the episode was called "It's a Good Life".
interesting fact, in the 2002 revival of The Twilight Zonet the show also contains a follow-up episode to the events of the original episode "It's a Good Life". Bill Mumy returned to play the adult version of Anthony, the demonic child he had played in the original story, with Mumy's daughter, Liliana, appearing as Anthony's daughter, a more benevolent but even more powerful child. Cloris Leachman also returned as Anthony's mother.