I'm going to have to agree, I think the scope of Animorphs runs just to wide for it to be contained within a movie, even a movie trilogy. A television serial, that managed to establish itself as a deep, exciting show with large amounts of relaxed humor, violence, and complex themes, would be successful. The series from a television standpoint would have to get into that depth a bit faster than the book series to begin with a hook. Most books would fit within one or two 40 minute episodes, and many books could be cut out. (helmacrons, the mutation, #39 etc...) Special TV movies could cover specials such as the chronicles and megamorphs, because although the megamorphs dont contribute to the plot, they are entertaining enough.
I would also consider aging the Animorphs just a little bit so that they are in High School, i think middle school age is too young to be universally accessible; maybe a bit younger than the characters of Heroes, which was mentioned earlier.
Special Effects-wise I dont see it being a problem considering the SPX of shows like battlestar galactica. The aliens would be a bit tough but definitely could be done so they dont look awful and cheap.
As far as cast goes i think we've seen that as long as the roles aren't too complex, teen actors step up to the plate. Despite all the negativity Disney stars get, the characters in those shows are portrayed fairly well. I think the show could work well as you got actors who were, at least, actors who could just not be awful. Mediocrity isn't a bad thing. The most important thing, even more important than the dramatic acting, would be the comedic timing and once that's pegged they'll be set.
Really if anyone were truly interested in producing this book series, it would be very possible. As long as its marketed right, has the right amount of action and violence, and definitely needs to have more romance to hook in viewers.
Yeah I rambled a bit but you get the idea. Animorphs.