Okay, none of the actors from the disney channel please.
You are going to need to find actors who are competant, which is hard for the younger generation.
Unknown actors are much better for the main characters, and then you have a supporting cast of decent adult actors. Personally I think they could bring Ashmore back to play tom, in the episodes where he was controlled, he played an excellent villian.
Also the Juno actress is a big no, she can only portray the emotions of your average stupid teenager.
There is a small chance that you are going to find an effective juvenile actor. If you saw Star Wars I: the Phantom Menace, Natalie Portmans first big role, then you know what I'm talking about: an inteligent fourteen year old leader of a planet who is constantly worried about the suffering of her people, but at the same time is highly curious and has a gentle human touch = difficult role.
The animorphs are going to be hard parts to play, even in the television series they couldn't get most of the characters right. Aside from Marco and Jake, the characters were done wrong. My theory is that the script writers for the television series either simplified the characters or they simply didn't read the freaking books.