I think it would be very difficult to make into a very good film because there are certain elements I think Hollywood would feel they had to cut which are, in my opinion, integral to the plot - things like the continued breastfeeding, which would be difficult to film with an actual 5-y-o, or even a young actor slightly older than that. People tend t obe wiggged out enough by breastfeeding (which is a shame) without seeing long-term nursing in films. If they cut that out, it would change a large part of the relationship dynamics between Ma and jack, and how their relationship is viewed in the outside world.
That said, I would love to see them try, so long as they were true to the story.
I also agree that this book would be difficult to adapt into a film. This is because the story is told through Jack and his specific vocabulary, so I think his perspective would be difficult to convey visually.