I have been choked out twice (intentionally), and being choked to submission is a fairly common occurrence in BJJ.
I'm a developer for a legacy system professionally, I train with a Surgeon in his last couple years of residency, several other Developers, and people in IT roles, two or three Attorneys at law. It doesn't seem to have effected our brains. BJJ started off as an adult martial art, and the academy targets adults, not children (there's one kids class, it's mostly the kids of parents that train).
There probably are a lot of shitty parents forcing their kids to extremes, for example shouldn't be making a 7 year old weight cut, that's a really dumb idea.
A properly applied choke held for less than 15 seconds, is probably not dangerous in the long term, a choke (or more technically a strangulation) held longer than that becomes substantially more dangerous for every second after. About 6-10 seconds in the person will lose consciousness, at about the 30 second mark the body will stop trying to breath for itself and CPR would have to be administered.
A 6 second choke isn't going to have the lasting impact of a concussion.
I've heard stories of people having heart attacks on the mat, read stories of people having strokes as the result of damage to their carotids, there's that poor kid in Brazil that was paralyzed in a match he had with an adult. There's always the potential for stuff to go wrong, but most of that stuff is preventable, and the stuff that isn't is usually the result of freak events.
That guy on ESPN Sports Science has been choked out by like 6 people now, ask him how he feels.