I don't know why none of those posters you were arguing with bothered to reply, but I guess I'll address some of your arguments. I hope they realize that most of them come across as sounding pretentious by claiming you're wrong without explaining themselves.
Clearly, the directors intended for us to empathize with the couple, but I only found myself glad that the lead character was behind bars as he wept at the end. The only time my interest picked up was during the chase/hiding sequence. I also couldn't help worrying about the welfare of the infant actor(s) as the actress held it while screaming and pounding the door, as the actors smoked cigarettes while holding it, etc. Maybe someone has to do it but I sure wouldn't let my baby be used like that.
The directors, who are also the writers, would be extremely stupid if they wrote such disgusting characters and attempted to buy the audience's sympathy with them. One poster accused you of being accustomed to Hollywood. Though I see his/her point, the poster did not explain himself/herself well enough. The implication was that, in the Hollywood classical style, the traditional protagonist is also the hero. Therefore, seeing the protagonist cry would likely merit the sympathies of the audience since, as the "hero," we are consistently on their side.
This film, however, is not in the Hollywood classical style, especially in terms of its characters. Bruno is the main character and therefore the protagonist, though he is no hero. I'm sure that I don't have to explain why this is so to you. So what was the purpose of the film, then, if the protagonists were horrible people? Maybe it was a character study meant to depict adults who are children at heart who are flung upon the world of "adulthood" way too soon. That's just an idea, other posters could probably extract other meanings, but I think this would be the general consensus.
Why do Bruno and Sonia cry at the end? I would say it's because of the circumstances they have found themselves in. Their immature actions have finally caught up with them, when it's far too late. They are suffering for the choices they have made. Ideally, how should the audience react? I think that if anyone felt mercy for these people, the directors would feel that their "message" was ineffectively communicated. They should see these characters for the people that they are. Here's another thought: my impression was that the two cry at the end simply because they feel sorry for themselves. This is only further justification of their characters.
The "baby safety" point has been discussed far enough. If there's something you wish to contend, please do not hesitate to reply.
"We played with life and lost
." - Jules et Jim
, François Truffaut.