Choosing a Martial Arts School
Do not be afraid to go to many schools in your area before deciding on the "Right School". (I recommend trying at least 3). These guidelines apply whether you are looking for you or your child. Just watch 3 classes (or part of them), and decide which seemed the best.
When choosing a school, first and foremost - study the instructor. Watch the instructor teach a class, talk with him, see how he acts and instructs. If you like him, the way he teaches, and the students he produces then you will probably enjoy studying at his studio. If you do not, then no matter how good the art is, the experience will probably not be a good one.
Secondly watch the students as they reflect on the instructor and the art. If the students are humble, talented, and friendly then that is the atmosphere the instructor probably promotes. If the students are egotistical, talented and "cocky", then that is also the atmosphere the instructor probably promotes (which good for competition and annoying for everything else). The worst case is if you feel the students are not talented, or they have an attitude that you could not tolerate (warring factions to be top dog, or a non-helpful atmosphere). Always rate talent relative to the students rank or time in the art.
The final thing to look for in a studio is the art it teaches. What is the arts purpose or what is it geared towards, and does that match your purpose? Some arts focus on tournaments, others on discipline, health and fitness, philosophy or self defense. Decide if that is what you are looking for in an art, for you or your child. Most arts actually overlap into many areas, but they also have one or two primary objectives. Find out what type of martial art you will be studying. Is the art you want to be studying a grappling art or a striking art, one that works with weapons or not, and what exactly do they require of you initially and at each rank? What is their goal and how does it mesh with yours.
Most instructors and students will try to convince you that their art is best -- and that that is the most important thing of all. I feel that your studio is like the company you work for -- sometimes who you are working with (and for) is at least as important as what you are doing. Almost every art will tell you that they are the "Ultimate" in self-defense -- ignore that. 90% of all automobile drivers think they are "above average" -- do the math! Every art is better than nothing, and none are as good as they think they are. If you can stick with any art for long enough you will get your knowledge base and wisdom on which to make further decisions. After a couple of years, and if you keep an open mind, you will be able to make these decisions on your own.
Chain or not
A lot of people will go into choosing a school with preconceptions on whether to go for a chain of schools or find the little Asian grandfather in a back-alley to teach them (only after they wax their cars for 3 weeks). Whatever. There's no good answers that apply all the time. Some chains have excellent quality control, or more importantly, excellent instructors that practice. Some guys in ally's in China Town are there, because they don't know how to teach or run a studio. And vise versa. I taught and practiced in both. If you pick based on the instruction and class, it'll work itself out. And again, watch the students.
If the black belts (or black sash's) look like they're spastics that couldn't escape from a wet paper bag, you might want to go elsewhere. If the students look like they can do material and follow directions, and they're getting something out of it, then your time won't be completely wasted, even if you move on later. Personally, there's a convenience factor (for parents or just practitioners) of joining places that have many locations. You never know where life will take you, and it's nice being able to continue your training with less variation.
So that's it -- prioritize based on Instructor, Student and Style. If you hate the instructors or other students (and their attitude), then you're not going to have a good experience.
After you make your choice, stick with it! Promise yourself a minimum length of time you will study the art (say 3 or 6 months) and stay with it. It is always hard to develop the habit of going to a new studio, as with anything new.
There are no shortcuts to chosing the right school or art. Every instructor beleives his Art is the best -- if he didn't beleive so, he would not have put decades of effort into becoming an instructor of that art. The students of those schools are also being programmed with the schools dogma. This is not as bad or as harsh as it might sound -- it is just a form of enthusiasm. Tollerate it, enjoy it -- but don't let yourself be programmed by it.
The Martial Arts are good for most people. I recommend them for almost everyone. But take your time, and choose the right school -- that will greatly effect your experiences with the martial arts in general, and probably define how long that you stick with it.