That was a good summation about teaching different Korean age levels. I found adults a bore as well but university level kids are still open-minded though some will come to class after a soju session. Keep things light and make a lot of jokes

Small children are the best to teach but not in a school where discipline is lax. I was fortunate to work at a brand new school with just a few kids per class each afternoon. As others have said attention spans are short so you have to think of a lot of activities (cooking, nature studies, science etc.). I wrote up my ideas after 7 years (articles/lessons section).

Some ESL teachers have written me directly to point out that physical contact is inappropriate with small children but I know this is just politically correct Western rubbish. To bond with the little ones you have to put them at ease with Great Big Foreign You.
Be prepared to hug and pat the kids on the head and back and learn basic classroom commands. Children want to be with people who show they love them.