Wow, that's was amazing!
I was amazed at the effort put into such a touching moment. You need some interaction, some violence and something funny. My suggestion is to put a back story to the grandfather. But overall the flash was really good, it has great potential for a follow up. I loved how the simplicity of a single visit touched your heart. I also would like to know the name of that song, you didn't state it anywhere. I like anime; especially it’s beautiful music.