I'm not sure about the Jr. version, sorry. And I also don't think the first poster has roles in the order you're requesting. Some roles are about the same, with stage time, songs, importance, etc. I would consider the 'leads" to be:
Baker's Wife, Cinderella, Witch, Baker, Narrator/Mysterious Man, Jack (and in that order)
Important but slightly secondary roles:
Rapunzel, Red, Two Princes/wolf (usually one of the princes play the wolf)
Smaller roles: Jack's mother, cinderella's step-mother, Florinda/Lucinda, Cinderella's father, Cinderlla's mother, Steward, giant voice-over, Snow White, Sleeping Beauty, Milky White. Most often Milky white is just a papier-mache cow or cut-out, but some productions have someone in a cow suit. ;)
Now, for the requirements of the roles, it depends on your voice type. Cinderella is a soprano, if you are a soprano you will be most comfortable with her or Rapunzel (who sings very high). Rapunzel is a small but still difficult role, in that she basically goes crazy and dies. The Baker's Wife is a large role that requires an excellent voice (mezzo/belt).
.....
Really, just try to get aq copy of the filmed stage show with the original cast:
http://www.amazon.com/Into-Woods-Bernadette-Peters/dp/B00001PE59
you might be able to rent it or get it from your local library. These are the "standards" for the roles, the measuring stick, if you will. Of course all productions have their own interpretations nut this should give you the best idea of what the "best" way would look like.