This Isao Takahata film, Grave of the Fireflies is about two children, Seita and Setsuko during the aftermath of the World War II bombing in Japan. After their mother has died from an air raid, they both live with their rude, cheating aunt before moving into an abandoned shelter. With their hunger and grief, both Seita and Setsuko are only able to entertain themselves with the fireflies at night.
If you haven't seen Grave of the Fireflies, I definitely recommend it. As well as a tissue.