Out of the mouth of babes is a saying for a reason. Kids don’t understand the social niceties of little white lies or saving people’s feelings. They go right in there with how they’re feeling and what they’re thinking, regardless of the effect on other people.
This can also lead them to do cute little things, like this: giving everyone little compliments as a sweet pick-me-up. Unless you’re the author of this depressing story. She obviously expected to get a note saying the same thing as everyone else. Imagine how she must have felt to know that her niece only “liked” her. I wonder what she’d done to deserve such a slight?