I work in a large chain grocery store, and this happened one night when a coworker and I had closing shift in our department.
I stepped into the back to get my water bottle, and upon coming out, a child came up to me and said, "Thank you for working at [grocery store] sir."
Then the boy went back to his mother in the deli section. This 10 second interaction made my entire day a thousand times better.
Until that point it had been a rather miserable day. Unfortunately, I did not have a chance to thank him, as he went back to his mother so quickly.
I know many of the stories you come across tend to be of ridiculous and annoying customers; but every once in a while you find a brief moment of kindness that somehow makes the entire day better for you.