I think the issue here is that a 6 year old can't possibly comprehend the dangers and consequences of this style of riding.
When a kid is old enough to make an adult decision - weighing up the risks and inherent consequences then sure, if they have the ability, let them go ahead. But you would have to have many long discussions about the potential consequences to ensure that they really did understand. You can rationalise it anyway you want on the Today Show, but I really worry about the motives of the parents. At best they are naive, and at worst they are opportunistic.
Do you honestly believe that if the worst happens and this kid ends up in a wheelchair for life that he will thank his parents for allowing him to proceed? It would be a high price to pay for 15 minutes of fame.
One last thing that worries me: who is maintaining the bike? Does his old man understand that it is operating way outside its design parameters and one mistake in preparation could end in disaster.
Good luck to him, but as I said before, what's next. If it is OK for a 6 year old, what about a 4 or even 3 year old. To follow the football analogy, if he shows promise then why not let him into the starting lineup for the Wallabies - he has the ability after all!! And we wouldn't want to hold him back… would we.