I don't really see the black belt as someone who can "defend himself/herself on the street," but rather someone who has been exposed to all the basic kata of the partcular art. This doesn't mean much since it's simple memorization for karate, Tae Kwon Do, and Aikido for example.
Ender Wiggins could have the whole lot of them in a fight...
To answer your question, Gwen:
Read the stickies.
Essentially giving a black belt to a child is pure marketing with no thought towards demonstrating real martial skill (the ability to hurt someone else and/or prevent them from hurting you).
Thusly we are faced with two possibilities: The instructor does not care if his students can protect themselves and awards advanced rank without this consideration; or the instructor is so delusional that he actually believes a black belt 6-year-old is capable of doing so.
Both scenarios tell the discerning customer to run screaming.
Thanks, scrapper, my thoughts and questions were geared toward someone else about a different dojo than this. But thanks all the same.