In the book and Saga it is made clear that Woltz is molesting his juvenile star named "Janie." We see her in two scenes: Right before Hagen meets Woltz for the first time, they are having a party for the kid and then in Woltz's mansion Tom sees the distraught-looking child who is swiftly taken back into her room by her mother who is not concerned about the molestation presumably as long as the kid gets roles. Later in Saga, Hagen tells Vito about this and Vito refers to it as an "infamnia."
Now I am not sure she was in any scenes in Godfather I or if she was referred to in that scene back in New York; maybe it was just in Saga. But IMDB does not have anything about the child or the child's mother listed in either version of GF. My sense is that this is deliberate, the actress maybe did not want to be associated with the role.
In fact, I have the vaguest recollection, no idea from where, that Coppola is keeping it a secret. Many people are still around from the film but this info seems still to be a unavailable. I would be interested in confirmation that Coppola is in fact protecting someone who wished to remain anonymous or any other info. Tiny details about the films are known, but not this one.