Sorry man, I went to bed. Only just read this now.
I like the trick of the story--the tragedy, I guess you might call it. It has weight and isn't a total cliché. There's a fair deal of symbolism you could mine out of the grandmother's character and I think the piece would really come to life if you expanded it.
Every time I've ever played with symbolism though I've been whacked on the head by people telling me to be subtle, so maybe be subtle about teasing out those symbolic angles or people will whack you on the head. Also, watch for expository or redundant sentences. The most annoying thing about writing is that people want you to tell them what happens, but they don't want you to actually tell them what happens. In first drafts or quick writing like in this subreddit, it happens naturally, but when writing larger pieces passages like "she appeared tired" or "I knew the answer already" should be cut out. The goal is to paint the scene so vividly the reader can see how tired she is. If you try to make a hobby or career of writing, you are going to have people say this to you over and over for years. If there's anything you "did wrong," I guess I would say it's that. But, shit, I'm just a dog on the internet.
If you've never read this short essay by Chuck Palahniuk, he says all this better than I can: http://cultureprn.tumblr.com/post/9176218270/thought-verbs-by-chuck-palahniuk Sorry about the awful centered font.
But like, for example, the image of a tired, older generation getting lost or left behind by the forward-marching younger one speaks to me a lot. That's a "big" issue and could hold up a whole story of these two people's relationship. My own grandmother doesn't have dementia but she's pretty useless anyway. The world left her behind sometime in the seventies. She lost her sense of wonder. I feel like that's at the heart of what makes us human, wonder. Without a sense of wonder, you are basically dead. No matter how well we live, or the meaning we create, one day we will lose our sense of wonder. There's real, vital emotion in that fear. A powerful story could stand on that foundational idea.
Thank you for responding to my prompt!