I’ll second your mention of the “Grave of the Fireflies”. I know Japanese anime is often very serious, it’s certainly not just for children. and the opening scene indicates the kids didn’t survive, but I kept hoping something would happen to save them, but their situation just kept getting worse and worse.

And although that story is set in post WWII Japan, it makes me think of all the other kids who died in a similar ways, in the many wars around the world.
And speaking of Japanese movies, I have seen the original Japanese movie that “Hachi” is based on. Even though I don’t even like dogs that much, I cried when Hachi is finally reunited with his master. It’s even sadder to know that Hachi actually DID exist, and DID actually wait for his master all these years. (AFAIK, there’s much less proof that Greyfriars Bobby actually existed; he may have, but the details of his story are rather vague and contradictory depending on the version.)
“Kid movies” usually don’t make me cry, but the opening sequence of “UP” certainly made me sniffle. That really was a rare “family” movie with a lot of depth.