i don't think the dragon was ever a child, he just took the form of the first person he say.
which happened to be a kid
It's a dragon. A spirit. Even more so, left to wander alone instead of being taught and guided by its creator - a river god. There will be no appropriate term by human social norms, that can describe what their relationship is. In summary, Taehyun was everything to Hyo-un. His world. His core because Taehyun was the first being that showed him kindness. If the river god was only able to teach Hyo-un, only then he would understand the true concept of family and lover.
But he also behaved like one so that's the main issue. If one doesn't have the social upbringing of an adult and looks and acts like a child is that not a child? Age is just one aspect of being a child maturity is another and sure he might check of the age box. But the maturity box is another completely
I can't anymore. It was all fantasy until "you are like a father to me sometimes, sometimes like a brother". I mean I get what he was saying. But that scene where while talking about sexual and romantic things in chapter 63, the flashback pops up when the dragon was still a child is where I draw the line.