M.L.
I'm a black belt and National Champion and have taught Tae Kwon Do to younger kids. Look at the schools, visit the classes. Most schools will let the child participate free the first time or two and you can observe the class with him sitting next to you as well. Look if the instructors are teaching respect, how to stay in line, discipline, how to speak to each other and the instructor.... some classic respect teachings would be taking shoes off at the entrance and bowing to the flag, turning around to fix a uniform belt, bowing for respect, lines in belt ranking... that sort of thing. I've been to a few where the kids are pretty much just rolling around on the floor, or sporadically sitting wherever, or the instructor doesn't even attempt to correct them... a very bad sign, especially since classes and set up costs can be fairly expensive. Classes like that are usually in the community centers/ymca's that sort of thing. Those kids would be better off in gymnastics.
Really though, most decent and honest schools don't have classes for children that young.