I understand your sentiments, the truth is apart from Harry porter or maybe Spy kid or even Karate kid I can't say I've seen exceptional children actors really it's very rare, and for such a story based on an amazing book they could have known that they needed veteran actors to tell the story very well, because if you ask me, I think they got some things right. And that children actor made the filmca bit mundane.
However because i seem to like the concept even if I don't know the book it was based on, I'd give the film a 6/10 rating.