Is it better to get a TESOL/TEFL certificate to teach adults or kids?

Well I would say you would be better getting a TEFL certificate for adults. The reason I say that is if you look at www.tefl.com (the most popular job website) you will find that most will ask for a CELTA which is to adults. That includes jobs to children only for some reason. Thus, I think it will keep your options open. For example I live and teach in Lisbon, Portugal. The majority of students are teach are under 18 but I needed a CELTA. On that note to be honest, if you don't do a CELTA (which I personally recommend if you want the better jobs), most other certificates won't state adults nor children specifically.

Yeah, it's up to you. It depends. Kids want to have fun. Adults sometimes want to have fun but, they are more interested in learning useful stuff. They don't always want to be in class. Maybe their job requires it or school requires it. I prefer teaching kids aged 6-13. They are a lot more fun and they are more open than most adults. Most jobs out there are for teaching children. But yes you can find adult schools, high schools, universities, etc.

It's true that most jobs just ask for a certificate in TESL/TEFL, and also true that most certificate programs don't teach you much about working with children. That's because historically most jobs for native speakers were with older students. But working with children is quite different. I highly recommend reading some books about working with children, even the books that are intended for ESL teachers in the U.S.-- Linda New Levine and Mary Lou McCloskey have an excellent book that you can get on Amazon. Classes tend to be large in other countries. I have been in elementary classrooms in China where there were 40-50 students. The students are very respectful- they stand up when you ask them a question, and classes are relatively easy to control. But it's quite difficult to give 40 students all a chance to practice English in a 1 hour class.

I have a TESOL endorsement on my BA degree and I can do both adults and kids with it. See if your college will allow you to add anything like that or add some classes in your elective pool.