Eaton was born on October 26, 1860, in Hartford, Connecticut and when he was eight years old, he moved with his family to a homestead in Twin Mounds, Kansas. That very year, his father, a Union Army veteran, became involved in a dispute with several Confederate men who had ridden with Quantrill’s Raiders during the Civil War. A short time later, six of these men appeared at their home, and Frank’s father was shot in cold blood right in front of the boy. Encouraged by a family friend to avenge the death of his father, the friend began to teach young Eaton to handle a gun.