-First of all, let me say welcome! Although I live in Denver I'm not familiar at all with any schools in Boulder. However, somebody here should be able to help you find something. 6th grader? 12-13 years old I'm assuming. Seems like a good age to get a kid into martial arts, especially if they like martial art movies. Learning a martial art really is like going to school, so most teachers should enforce concentration for students. The big part is not necessarily finding the right CMA, but the right teacher for your cousin. Its worth it to check out multiple schools, see how they run the classes, see if they have different programs, the price of classes, how many times a week to attend, signing a contract if necessary. There are plenty of threads here for you to look at and gain some perspective about choosing the right place. Most importantly, I would say, is to talk to the head instructor of any place you go, plus take a good look at the senior students, for they will be the best representation of the instructor's quality.
-Not to bombard you with so much, so therefore don't hesitate to ask questions. So many people here are quite insightful and are ready to help make a martial arts journey possible.
Best of luck!
Andrew