The thing you have to understand about Grave of the Fireflies is... it's an autobiography. The writer is ashamed to have actually survived, so treated the fictional version of himself the way he feels things should have gone. That puts a whole new spin on the story.

For a different treatment on the subject, go watch Barefoot Gen, an autobiographical cartoon about surviving the atomic bomb. More uplifting than Grave of the Fireflies, to a very surprising degree.

I have the manga, or at least the first parts. The movie basically seemed to say, "yeah, getting nuked is bad, but hey, keep a positive attitude!" Safe to say, it's creator came out of WW II much happier than the creator of Grave of the Fireflies.