Rose, Dixie does look like a possible Polly, or perhaps Jill.
PrinceRilianIX I do like the look of Rafe for Frank, he seems about the right age and looks serious and down-to-earth. Olivia might be a little too striking for Helen though - I've always pictured her as very normal-looking.
Twigs I could see Pixie Davies as Jill, though I haven't watched the new Mary Poppins so I can't say anything about her acting ability.
My own suggestion is
Bronte Charmichael (who played Madeline Robin in Disney's
Christopher Robin). She is also about 12 and I initially thought she'd be a good pick for Lucy, since she came across as young and innocent in the movie. However she'd probably do a great job as Susan (she reflected a responsible and mature side in the role as well). Especially given her age and that it'll be at least a number of months before filming starts. She is a British actress, which helps with the whole accent question.