You have the start of a good case that “A Very Old Man with Enormous Wings” is not a children's story because of its high-level vocabulary. However, someone could argue against you that children actually have to build up their vocabulary and comprehension skills by reading stories that are challenging for them and by learning how to figure out words from their contexts and/or look them up. This could be a possible counterclaim against your argument.

You can strengthen your argument by getting a bit more specific about the age group of the children you are referring to. If you mean younger elementary school students, you are probably right. The story does contain vocabulary that is over their heads. However, if you are thinking about students in middle school, this might be the kind of story they need for a challenge.

You may also consider mentioning the themes of the story as well as the vocabulary. Children may not, for instance, be able to grasp themes like ambiguity and the difficulty of interpretation. They may also not understand the reasons behind the villagers' abuse of the old man or why Pelayo and his wife put the old man in a chicken coop. This is really quite disturbing. Finally, children may not be able to understand the depth of symbolism in the story. Including these points in your essay will further strengthen your argument that this is not a children's story.