People we don't even know the names of yet. Ideally. Average suburban middle-class not-perfect-looking-models normal everyday kids.
If you've heard of them before or recognize their faces, they're automatically wrong for the part.
In this case, believability is law. That means no Disney kids, that means no one who's acted in anything with recognition. As much as I loathe Harry Potter, that's something major they got right with their adaptation.
I'd even go as far as saying a no-name supporting cast (parents, teachers, Tom, Andalites, etc) would be preferable. Maybe get someone like Ron Glass for Elfangor or the Visser, the recurring big roles, someone a little more recognizable but not high-profile. But the kids would absolutely have to be "and introducing..." in the credits. Example, Edward Furlong in Terminator 2. Absolute no-name.
The problem with Animorphs casting is aging. These kids are between 13 and 16 in the books. A movie takes two-three years to produce. One movie, three years from scripting to marketable product. So, you start with a 13 year old in pre-production on film one, by the time film two has its theater release the kid is already 15-16. You can maybe make two movies, tops.
Insurmountable challenge, unfortunately.
Animation is obviously a different story. Either traditional or CG, doesn't matter. Voicing doesn't pose the same problem as live-action, live-action is pretty much a "don't go there" scenario, if the responsible party is wanting a reasonably faithful adaptation of the source material.
