My son was asking about her and I had to explain why I was sad yesterday. I didn't want to use "went to sleep" or any other euphemisms because that would confuse him, so I outright told him that she died because she was very very very old. Fortunately, he didn't ask what "died" was yet and left it at that.
I hope I made a small difference in her life at the end. I think I did, and that thought makes me feel a bit better. She had almost no one visiting her: she never married, had no children, and most of her friends and distant family lived out of the area except for a nephew who visited when he could. But she had me every Friday for a few weeks.