Jack's sacrifice, one could argue, allows us to see that he is no longer the cold hearted callous man as he was in 1965, when he agreed to play a part in starting the whole mess. Going further, insofar as he is responsible for feeding 456's addiction in the first place, without any moral regard, it is fitting that this unforgivable act would cost him his own grandchild's life in the end, and leave him a broken man. Call me judgemental.

Indeed the whole repugnant mini series, filled with allusions to child violence with a child sacrifice at the end, fits together very well as a cogent story. However, It's still a repugnant story that I wish Davies had done differently. Obviously, the Doctor can't come to the resuce in every crisis, even if Jack is at the center of it — and this is made clear in the final episode, but using children in fiction this way — well it's something the 456 would do.