This is a poem by Yeats. We can never be completely sure what the author meant, but clues in the text suggest at tragedy. The child is entreated to come with the fairy--to escape into freedom, to leave behind everything he knows or ever found comforting. Something must have happened to tempt him into this, because he accepts their offer, and leaves--but he is not joyful about it. His eyes reveal that he has witnessed some terrible things beyond his comprehension.
Perhaps he is an abused child being led into the freedom of death by the spirits of the wild.