If you just focus on the guilt question you have understood nothing..
The movie is about the horrors of war. People suffer. Who is at fault for each individual suffering is completely secondary. Could the kids have survived if Seita had taken different actions ? Maybe. Was he too proud to go back to his aunt ? Maybe. One way or another it would not have been easy since as the farmer says its not about money or Kimonos but about the lack of food in general. But that all is not the point. If you leave the movie being angry at Seito for not acting differently you have not understood the movie.
War itself is the root of all evil. But its also wrong to focus on things like pearl harbor or who started the war. The whole point of the movie is to display human suffering at wartimes (especialy among the civilians) and that that suffering is brutal and cruel. Even if you think that Japan is at fault for the war and the bombings (thats fine) and even if you think that Seita should have acted differently you cant possibly say that the kids deserved what they got. Noone in his heart of hearts can be that cruel. In my eyes "Grave of the fireflies" is probably one of the most effective trials against war that there is and certainly one of the strongest appeals to human emotions I have seen in any movie.
Full disclosure: I am german. My grandmother used to tell me stories about the war and the immediate time after the war ended. To make it short. It was rough. She used to say: "Who experienced this war does not want to have another." That in a nutshell is the essence of the movie. A message that should never be forgotten.
First post I made on imdb since years. The overall focus on the guilt questions in the topics just angered me.