While other's mind was immediately centered on something else, the first thing that came to my mind is worry. First, the dragon doesn't remember Taehyuk. Then, Hyo Un remembers him, but Taehyuk nearly dies. Then, Taehyuk forgets, gets abducted, and gets thrown to the sea. Even their past life is tumultous. Just when things were looking up, he's suddenly seized by a strange sickness which made him turn into a child... Considering the seriousness of the problems they faced, I'm just worried something terrible is brewing in the horizon.
Thinking about it more clearly, I am probably worrying too much. Taehyuk now has the orb, and their memories are restored. This could be nothing more than a small hiccup, perhaps an allusion to the fact that Hyo Un was once a child spoiled by Taehyuk along time ago. Maybe, like someone else said, he just wanted to relive his youth subconsciously, and wanted to be taken care of. Hopefully it's not too serious.
I guess what was really off is the timing of when this event took place. It could have happened before, or after... rather than when they're on their honeymoon. But, maybe the author will reveal the reasons why it had to take place now and how it should fit in the story.