While it has a similar ‘look and feel’ to Studio Ghibli, never make the mistake of thinking that the movie Grave of the Fireflies (IMDb rating 8.5, 446 reviews) was done by them. It is one of the most painful tragedies ever made, and even a lot of adults are traumatized by watching it.
But it is one of the best animated movies ever made.
Princess Mononoke is scary enough for me. Very graphic.