bobfells wrote:I believe that Buster Keaton's baby son played his character's son in OUR HOSPITALITY (1923).
No, Buster's real-world son plays his screen character as a baby in the prologue. So this is actually the opposite twist of most of the films mentioned so far: an actor and his child playing the same movie character.
Douglas Fairbanks plays men from three or four generations of the same family in The Mollycoddle, though unlike Don Q they don't share screen time.