I believe so...My Neighbor Totoro was paired with it when it was first released in theaters because the studio believed that no one would see Totoro on its own. Hotaru no Haka was a novel first--partly based on the author's own experiences as a child--so that may have been why they expected it to be the bigger success. The fact that they'd pair the two at all for a double-billing suggests it was being marketed at the same audience.
Melancholy and even sad endings, character deaths and the like are more prevalant in anime than western animation, even in stuff aimed at children. All the reading I've ever done on the culture talks about the Japanese stress on the briefness of all things material: life is at its most beautiful when you realize the transience of it (check out poetry on cherry blossoms or autumn leaves). It's a more natural way of looking at things, I suppose...death is accepted as part of life, rather than danced around or hidden as it tends to be in the west. It's one of the things I found appealing about anime: it's much more up front, more respectful of my intelligence, because once you hit a certain age you know anyone can die. (No infant immortality in real life, and I do like my fantasy to maintain a certain level of realism regardless of how many catgirls and giant mechs are in it.)
Not that any of that makes Grave of the Fireflies any less depressing. Though actually, rewatching it years later...since I knew what to expect going in, I was able to watch it more critically (and I was doing a paper for a class, so I kinda had to. ^^') It's still a sad movie, and Seita and Setsuko's story is still tragic, but the eventual outcome for them is that they're together again, and nothing can hurt them. There's the possibility they'll be reborn into a better life. Melancholy again. Beautiful and sad, and just a bit uplifting, strange as that sounds.
(By the way, if you're looking for a movie that pairs well with Grave of the Fireflies, I'd recommend Akira Kurosawa's
"Rhapsody in August". Yes, live action. If only for the oddness of watching Richard Gere deliver lines in Japanese. More upbeat than Hotaru no Haka--which is to say, not unbearably sad 90% of the time--but still deals with the ramifications of war from a citizen's point of view.)
