On the one hand, I think it's great to start them early so they can develop the judgement and movement habits that will stand them in good stead during adult life... I wish I had started earlier myself.
On the other, I've known a few young martial artists who lacked the mentality of responsibility and abused their abilities to damage someone else, and while they were lucky nothing serious happened, it could have been nasty.
I also find many people who start young get a more 'cushioned' form of training (because hurt kiddies is bad) and so well into their more adult careers still habitually pull their punches, often to their disadvantage. Learning to physically pull a punch when necessary is easy, but learning to apply power when necessary is the hard part.
Where possible, I think it's best to start them off at a stage where they can go in with some decent power and habitually make their hits count when they need to. By this stage they usually have an appreciation for the consequences of fighting and have enough mental control and discipline to pull their punches when they need to.
Just my thoughts, though.
Can't see vid here at work, but will check it out from home!