Even though they deny that it's a given, I would expect Helena Bonham Carter to get a role, if Tim Burton is directing. I don't know Juliet Landau, so I don't know how she would compare with Helena as Julia, but I can't think of another role that would suit Helena. Of course, I could be wrong, and Tim might actually do a movie without her
I don't know many child actors, either. Doesn't Dakota Fanning have a younger sister? The kid in Sweeney Todd, Ed Sanders, was quite good, but he seems to have grown about a foot and lowered his voice an octave or so since filming, so he might already be too old for David Collins.
I've heard Hayden Panettiere (Heroes) mentioned for Carolyn- I could possibly see it, but wonder if they could get someone a little more flashy.
About Vicki's age, wasn't Joanna Going's version supposed to be 25? I think that's a plausible age- someone who has been college educated and may even have a master's, but is still pretty young and fresh in the world. Making her in her mid 20's would seem to open the door for more possibilities actress-wise, though I can't really think of anyone at the moment.