Living in Korea,
You wrote that a good mix of adults and children is needed to avoid falling into a rut. For me, though, I don't think this holds true. Like any job, you (one) need to set new goals with which to challenge yourself (oneself)--regardless of which age you (one) teach.
I used to teach JHS students when first in Japan, and kept the job for three years. It was great, but I also knew when it was time to move on. Now I prefer adult classes because their life experience can be incorporated into the classroom. I also like using current events (or making references to what's happening in the world today) in the classroom. Hence my website, which also provides new challenges for me. I couldn't (and wouldn't) want to go back to children, with the exception of teaching my own daughter English.
I agree with you that a mix is important, though, especially when you start out teaching. Because I taught in the public school system in Japan, I understand more about the education system here. It translates quite well into what students know and don't know, their strengths and weaknesses, and all-around structure of learning for adults when they later return to study the language. There's a lot of experience to be gained from different age groups and different levels, which can be used in unforeseen ways later down the career path.
Chris Cotter www.headsupenglish.com