Grave of the Fireflies destroyed me in college. I was ~20 at the time and cried like a baby for a solid 10 minutes. I'm now a grown man and I'm not sure rewatching would yield different results.
I saw it as a teenager/kid (~16 years old) and it made me deeply sad but also more connected to people who have to experiment the atrocities of the war. I don't think it's bad for kids if they are not too young, it's not an easy movie but you can learn a lot from it.