Please help i need to know/find this movie.. Please please please help if you can?

Ok im looking to this Japanese anime movie that is based on the aftermath of the WWII bombings of Japan. It is centered around a boy and his sister and they go through japan and see the destruction of it and their mother is completely wrapped in gauze and she dies with maggots feasting on her (awful i know) and the story is being told by the boy and shows him in some underground place and 2 Japanese officers find him and says "he's long gone... you can tell by the look in his eyes"... or something along those lines... and that is basically all i can remember... please if you have any idea at all or even the slightest clue it'd be gladly appreciated