If the students are humble, talented, and friendly then that is the atmosphere the instructor probably promotes. 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. 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. What is their goal and how does it mesh with yours. 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.