My kids (8 and 6) snuck out of bed and ended up watching Grave of the Fireflies with me. I originally didn't watch it with them because of the subtitles, but they were hooked.
They were hooked until the end. It was quite late for them, but school vacation so I didn't push them to bed.
I didn't know the story going in.
At least if I was their age, I wouldn't have reacted to the end. I didn't become sensitive to tragedy until I was in my 20s, except that I cried when I read Bridge to Terabithia when I was 10 or 11.