Responses

Based on my belief, I think Hyo-un took on the form of a child, his a dragon who saw a child and took on the form of a child to blend in. He just awakened but was like a defective dragon who never got the training done to fully awaken as a dragon, if possible in a grown human form, so because he was confused and tried to belong, he saw a human girl and took a childlike form, If he saw a human adult first he would take on that adult like form. It's a bit weird but it's understandable.
I don't know how to feel about Hyo-un with Taehyuk in their previous life. He is a child, but he grows rapidly to look like an adult.. And the master was the one who was raped.. But he is still a child :'( So I am going to tag it with Child Sexual Scenario and abuse anyway, but not with grooming since Taehyuk did not have such interest. Looks more like Hyo-un groomed Taehyuk instead. Could have done without the rape scene, but he is a child and a dragon and in heat. This is so morally confusing since I don't know what's the age of consent for dragons.
This has a long flashback but a good example of how it's done right. Not talking about story but the transition and pacing.