I want to know who is responsible for the little boy's death?
I had to pare the question down. I encourage you to repost the second part of the question. I think that the family, itself, ends up becoming responsible for the boy's death. In the end, the husband and wife become so paranoid of things and so driven by fear that their home becomes more of a fortress than anything else. It is this reality that fails to account for the childhood innocence of the boy, choking the life out of him at the end of the story. The family's obsession and fear with the outside world, and their own sense of selfishness that nothing should remotely take away from their own belief of entitled happiness helps to do terrible things in the end. The growing paranoia results in the building of a high wall, that the boy sees as an obstacle to climb. The fear of the outside world compels the family to add the jagged edges to the top of the wall and the barbed wire enclosure, that the boy sees as another hurdle for his own vision of Prince Valiant to overcome in order to rescue the maiden in distress. The family's construction of these safety measures without acknowledging the fear and insecurity that are needed to live in the modern setting is what ends up taking the life out of the boy.