i thought the acting done by the childern was well done. it seemed rather natural than rehearsed.
There was only ONE tiny little flub on that.
Right after Aslan is killed on the Stone Table, the girls are sitting by him. Lucy just barely looks up and throws a glance to someone offscreen to see if she's doing it right as she lays her head down on his sholder and starts crying.
It's a tiny little thing, and if I hadn't been so utterly absorbed in how real everything seemed, I'd have never been paying close enough attention to it to have noticed.
Aside from that one point, I never saw anything that made me think it was anything less than looking out a window and watching the action unfold live.