Whilst not the most historically accurate book (by far) I think the overall emotive feeling is really what is important about this book not providing a history lesson.
The general ignorance of Bruno possibly reflects the ignorance of the people during the time of the second world war. Not that people did not know what was going on I imagine a lot was blanked out in people's minds, not wanting to know what they see and saw going on around them.
The overall emotive story is more important and I do not think this book at all trivialises the holocaust although I can see how you could interpret it in that way.
Boyne strips the history down to its bare bones, leaving shapes rather then details. All in all, this makes the ending much more effective then had it been a more accurate portrayal.
People have become desensitised to such horrors - we are too used to horrific violent images that we become used to it and the emotional effect is dimmed partly because we expect it to happen already. Maybe out of self-defence.
At least with this book, our senses are not over ridden about how horrible it would have been. We already know that. We should already know what the holocaust is like, what happened, how it started, how it ended, we should have already seen the pictures and learnt about the second world war.
There are all sorts of books. Some are more educational then others, some can describe in minute detail what everything looks like, what happened - the reality. Some provide a good history lesson at the same time as being entertaining.
Some, like this one, make you feel something the others could not - something a lot more simple.