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. Here is some information on what to expect from different cultures and arts. This information can help you choose as well. Once you realize that System or Style of art is not the only priority, then read Choosing a Style .