Dragon is a child in adult body

Not really. In the past which is the recent chaps he is considered a child. But after more than 300 years how could he remain a child? He has grown in all way expect he's only childish with him. It can be seen in the first few chaps. He's a grown man to all.

I chose to believe that Hyoun was more of a teenager when Taehyuk found him Hear me out:
-It is said that when his mother/creator died, he was wandering in the forest and by the time he already had consciousness. -Who knows how much time passed while he wandered between his conscious and unconscious periods.
-He looked like a child not because he is one but because the first living creature he saw was a child.
-He acted like a child because everyone treated him like one. He also raised himself and was neglected, so he grew up immature as being rude, impulsive, a crybaby and really possessive, we can see he still can’t control his emotions after 300 years. He also gets too attached by receiving minimum affection/attention.
-After making the villagers disappear, he was alone for at least a year if not more as it’s said several seasons passed while he was asleep.
-Finally, Taehyuk does stay with him for a few years, as he is not surprised (until later) that Hyoun has grown up.

Dragons age differently. We can see the new dragon in this latest chapter calls him "so young" "my child" and a "newborn". He's young and lacking in experience even for a dragon, and spent most of his time in an unconscious form where he wasn't able to mature. The short time he was with humans was as a kid. Nothing about him is adult.

In thr past at least.

That's the point we can't compare him with human ageing. He can literally decide to be whatever he wants and for that he needs knowledge which apparently he lacks. He's a newborn not in the sense of a child.

What other sense is there lol. To gain that knowledge he needs life experience which he lacks. Thus making him an immature child.

Even a man can lack knowledge in. an adult body. And a child can mature enough to understand grave situations. I just gave you an example "Ryeo-ah"