Certainly the most important character in this story is Francie, so casting a talented child actress would be key. Francie is sensitive and intelligent, likable but also socially awkward at times. I think someone like Elle Fanning would be perfect as she is proving herself to be very gifted and able to give powerful and subtle performances. The part of her father is also very important, since he must be sympathetic, but also flawed, since his drinking and shiftlessness cause problems for the family. Also having him be of Irish descent seems important to the immigrant context of the story, so I'd suggest Aidan Quinn for this role, who is of an appropriate age and has a warm, manly quality.