Adults! Definitely adults unless you really love kids AND have the patience of Job.

It really depends how old the kids are. from 0-6 years, unless you are born to love kids and have endless patience, they are a pain in the butt! when you want to teach with play, most parents don't agree because it's still the old indoctrination of study. and those classes are boring for the kids, so they scream and yell and only say: 不幸 or even worse 不行！i can't do it!

then you have the 7-10 year olds, they are actually really cute, some already speak English, but in order to cover the massive holes they have in English (as it wasn't properly taught in school) you're basically starting from scratch, teaching them mostly phonics.

the 12 and up kids, are pretty cool, in my opinion, some have knowledge of English and other things. so you can actually have a discussion with them.

the high school and university students vary hugely, depending on their upbringing. some are diligent and interested others just want to pass some exam and don't really care for English. again it's about who you pick because you'll be spending a lot of time with them. Good Luck.

To add to LiangMingZhu, adults may be more motivated, as they are generally choosing to be there, where as anyone under 20 is usually being told to be there. Also, adults will be more likely focused on one particular aspect of business - more likely business English.

If you want money - kids - but BOY will you earn it!

If you value your sanity over salary - adults.

Learn to love what you're doing, never complain about how hard is and think about what Mark Twain said about his mother: "My mother had a great deal of trouble with me but I think she enjoyed it!" That's become my credo since I started to teach little kids here. Believe me, you will enjoy it especially when you learn to be patient, patient and patient. Yes! Make them happy, let them feel happy at learning English. If the kids like you they will learn easier the language. This is how I learnt my first language when I was 12. I simply found the teacher such a great person that from the first day I've fallen in love with the French language, not the teacher and so far it's still the language I love the most. From the very first day l memorised all the vocabulary and the questions and answers related to that vocabulary. I wouldn't say the same about German. The teacher was a sort of cold person though still a nice one too. It took me longer to learn it but I made it not for her but because German it's a nice language too. I am aware of how strange it sounds to my children the English language, but when I see them laughing and yelling when they repeat my words, I feel happy too. I know they may not learn everything I teach them right away or later but they will love English because their teacher was patient, understanding, funny and honest with them too. Nev exaggerate ate anything about yourself, avoid anger at any costs, keep voice as calm as you can or more than you can. Kids deserve the best and you can give them this happiness that mostly they don't get it when they study with their Chinese teachers.

I am usually teaching both: adults at prime job, and little ones (1.5 - 5Y) as part time (less than 10 h a week).

I always have fun with kids, because they are very direct. When I am alone with 10 kids, I imagine it would be the same as if I would be abducted by UFO:

- aliens would speak language unknown to me;

- they would poke their fingers into all holes on my face;

- they would all jump on me, sitting in the plastic chair, and watch how (if) I will survive the impact;

- there is an interesting 1.5Y old boy, who doesn't really want to learn or talk English. He is always armed with wooden stick or similar tool in his pocket, and when he's tired of listening English, he comes closer to me, and try to dismantle my tape recorder, and so on. 

I always leave class with babies in good mood.

  I didn't teach kids until i'd had a couple of years experience as a teacher, but then I found I loved it. Personally though I could not do it without a huge amount of flashcards, a huge resource of games, a decent level of Chinese or a classroom consultant and a well worked out reward scheme to bargain their attention with when your lesson plan falls short of the mark. Oh yeh, and an absolutely endless amount of patience. Hopefully though, if you have the first things sorted, you shouldn't need the last one.

It all depends on your style.  I find once you get the ropes of teaching English to kids and they get used to you its fairly easy.  As the above poster mentioned, some things are absolutely required though.  Like a solid amount of games they like (if you have more than 10 in your inventory you can make it), a good and fair reward and punishment system and lots and lots of patience then you'll go far and may learn to like it. 

Teach both...a very good game to play with kids is a game called charades. It requires action for the other kids to guess what that kid is doing. Play that games with kids they love it. Also, you can do an activity which I love and that is draw the face. Have one kid come up and draw one part of the face such as the eyes. Ask another kid to draw the ears etc., After they draw a face you can illustrate it using your own words or you can create something out of that face and teach that word. I use balloon!