My kids say this some times. Then, I try to tell them they don't know anything about unfairness and tell them how some families struggle to get enough to eat.
Just now, Devin said something was not fair. I'm not sure what. Here was Trevor's response.
"You know what's not fair? Some people don't even get enough to eat!"
I wish this meant he really understood just how unfair life is. But, I think he was just repeating what he had heard. From me.