The reason that The Ickabog is a fairy tale for children even though it has dark elements, is because dark elements are a fact sometimes in many children's lives. In Elena's review she stressed that she could not possibly know how the story was meant to cheer children up. It is not necessarily written to deny or lie to children that they must pretend that they are living in a perfect world, when the majority of children in the world are not living in ideal circumstances.
It was written to give children a sense of life being an adventure, and difficult circumstances as a part of life, and something that faith, and spirit and perseverance in going on a quest to battle the imaginary monsters inside of the minds, that persecute them, that Elena referred to as heroic hogwash, just goes to show that she prefers to see only the negative side, without seeing the benefit of overcoming the monsters.