Because they were raised as royals since birth, while MC is an ordinary man who was suddenly put into a drastically different situation with the threat of death hanging over his head and the responsibility of being a king, while he barely got 5 minutes to settle and think ever since he got isekai'd. I think that he is doing pretty well, given the situation.
Yep... not to mention discovering his sexuality and all... I mean... just the cultural chock of the different type of clothes and manners... for a common person to suddenly become a king and having to meet another king... it's a lot of pressure... and being an adult is nowhere enough to offset it... ┑(￣Д ￣)┍
damn. why are the children acting more like adults than kenneth tho