Major problem with the ending

***Spoilers***

I think it's way more complex.
The film is not just about surviving, but also about panic and insanity. From the very beginning, the two main characters are surrounded by people who've lost it, but seem to be a lot happier, because their deluded minds refuse to accept the truth and creates a new world. Ultimately, who are the insane in this new world? Reason and sanity are always defined by the most common denominator. Here, insanity and panic have become the new norm.
So in a world where only two people are still sane, when the rest of the world has gone bonkers, what is there left to do? Either you become like the rest and "embrace the darkness" or stand up and fight. Breathing the ash is not the only puzzling thing: surviving the train wreck; going for days without food and water; experiencing a car crash; falling out of a helicopter - all this could be seen as plot-holes, yes.
But think of it this way: what if the children are being challenged by the forces above them (a kind of Greek tragedy)? In a world where people have gone insane and where hope is lost, those who still have it are rewarded by surviving adversity.
By the same token, you can also see them as being cursed. When there is no hope left and when everyone has gone insane, there is no reason to carry on. You can just let yourself die, as it would be the simplest way to end the nightmare, but some forces have other plans and want you to keep going.
In any case, it's a pretty mysterious film with lots of possibilities. Having said that, the very last line spoilt the ending. Really silly.

'Marxism is the opiate of the unstoned classes' - Art Kleps