Sonny, I believe she appears twice in GF II: once as the child on the boat, once as young Mary. Here's the Wiki entry:
The Godfather Part II
Mary first appears in The Godfather Part II as the youngest child of Michael and Kay. She is a young child (aged about 4 or 5) in the late 1950s. Like Anthony, she does not have a significant role or story arc in the film.
Unlike James Gounaris, who played young Anthony, she's not listed in the credits.
Sophia Coppola was a toddler by the time of GFII, the young Mary seen playing in the hall way during the fight between Michael and Kay strikes me as being a few years older.
