I didn't remember it either, so I went to imdb.com.

From the trivia for the 2003 episode Twenty Five:
While Toby is in the hospital room with his babies, the television is playing what is supposed to be home movie footage of Zoey as a child with her father. Even though the younger Jed Bartlet in the home movie is only seen very briefly, he is played by one of Martin Sheen's real-life sons, actor and director Emilio Estevez. This is the second time in their careers that Estevez has played a younger version of the same character that Martin Sheen was playing; the first time was in Nightbreaker (1989).