Yes, it is sad.
Grave of the Fireflies is quite possibly the saddest animated film ever created. Isao Takahata, the producer, has done a wonderful job of painting the life of the Japanese during WWII. However, it is a very real picture of what happens during war. The movie is based on the book of the same title, by Akiyuki Nosaka, a man who lived the war. To produce the movie Isao read the book and interviewed the author to help bring the movie to life and find the true intentions of the author.
Isao's intentions for Seita were to show a young spoiled boy who is thrust into a situation where he needs to grow up quickly or he will become "worse off" than other children. He planned for Seita and Setsuko try and survive the war not by dealing with their issues, but by creating their own fantasy world where nothing could harm them. Unfortunately, reality caught up with them.
It is a wonderful movie that shows how making good choices and taking responsibility can make a huge impact in one's life.
A note on the violence: This is a war movie. Yes, it is animated but is still very much a war-time film. Please, exercise caution when showing this movie to young adults or children.
This title contains:
Positive role models
Violence & scariness