I'd want nothing but unknowns are the roles of the Animorphs because our current generation of child/teen actors have not proven to be anything even remotely talented outside of disney/nickelodeon (though I'll give credit to the iCarly cast. They succeed in actually being entertaining)
If Animorphs goes the animated route then we have:
Jake:
Yuri Lowenthal,
Johnny Yong Bosch, or
Steve Staley (Sorry. Couldn't find any good clips for his voice)
Rachel:
Michelle Ruff,
Mae Whitman, or
Cherami LeighTobias: Yuri Lowenthal, Dave Wittenberg (he's another one I couldn't find voice clips for. Try and find watch his portrayal as Henry from the third season of Digimon if you're interested in what he'd sound like as Tobias),
Jesse McCartney (listen to him playing Nightwing from Young Justice and Roxas from Kingdom Hearts)
Cassie:
Cree Summer, Cherami Leigh,
Danica McKellar (Listen to the clip from Young Justice), or
Masasa Moyo (listen to her as Bumblebee in Young Justice) (Incidentally, she's already played Cassie once in the PC game)
Marco: Yuri Lowenthal,
Todd Haberkorn (especially in Rosario Vampire, Fairy Tail, and Fullmetal Alchemist),
Vic Mignogna, or (if we're going the Avatar route and having actors who are the same age as the characters they play)
Logan Grove (Beast Boy)
Ax: Todd Haberkorn, Steve Staley, Dave Wittenberg, or Vic Mignogna