Ooh, thanks for the character descriptions of Kerd, we should be able to find someone to suit that role...*thinks*
Now, Unys - I imdb'ed Cate Blanchett, and found movies in which she's played a mother, and voila, there's a girl who played her daughter in The Missing,
Jenna Boyd who certainly looks as though she could pull off the snobby little brat Unys :P

she's currently about 13.
Ooh, and just had a crazy idea...what about
Toni Collette for Duran?

She's the sort of person you definately wouldn't want to come up against in a fight...
Ok, talk about 6 degrees of seperation...Toni's imdb lead me to
Nicholas Hoult. Could he play Kerd, do you think? He seemed really sweet in About A Boy.

Gah, dodgy photo. But aren't his eyes *blue!*
He's about 15 at the moment