Kristin Chenoweth is waaaaay too old for Evelyn. Actually, she could make a good Mother, possibly (though she's rather old for that role as well.) Remember that this is film, guys. Brittany Murphy would make a fantastic Evelyn (and judging from what I've heard from the movie Happy Feet, she can sing!) I like the idea of Toni Collette as well.
Nicole Kidman hasn't the upper register for Mother and as others have said, her voice isn't strong enough.
Mandy Patinkin is, as has been said, too old for Tateh now. As is Robin Williams.
LOVE the idea of NPH as Younger Brother.
What about Tracy Thoms as Sarah, possibly? From what I heard in RENT, she seemed to have very nice high notes.
Jimmy, what are you doing here in the middle of the night? It's almost 9 PM!