The Window (1949)

When young Tommy (Bobby Driscoll) sneaks out of his bedroom and onto the fire escape of his tenement building, he sees two neighbors, Joe (Paul Stewart) and Jean Kellerson (Ruth Roman), murder a sailor (Richard Benedict). Tommy, who has been reprimanded before by his parents for telling phony stories, informs his mother (Barbara Hale) of the crime, but she does not believe him. After his mother leaves to attend to a sick relative, Tommy worries that the Kellersons will try to kill him, too.