Since hyo-un's form changes depending on his unconscious will (his only true body is that of the dragon), I believe he turned into a child because they're in a place that looks like where they lived together when he was in child form. So, he's subconsciously gone back to said form to fit the image he has of the location.
Since hyo-un's form changes depending on his unconscious will (his only true body is that of the dragon), I believe he turned into a child because they're in a place that looks like where they lived together when he was in child form. So, he's subconsciously gone back to said form to fit the image he has of the location.