like the name of the place they move to in his mind is called "outwith" which is his mis-pronunciation of Auschwitz

Is that right? They didn't give any name for it in the film. That's just as well, actually, because it would have highlighted the inaccuracies and spoiled the effect.

(it becomes apparent what has happened but I wonder whether the movie shows more than the book describes..?)

If you do see the movie, I'd love to know how the ending compares to the book. You don't see it directly on film, but you see enough that it's impossible not to get it. And then it switches to the adults' POV as everyone realises. It's pretty gut-wrenching; might be easier if you know what's coming though. I didn't.