Teenage boys are killed every week on mopeds and motorbikes. It is perfectly legal if you are 16/17 to drive these things. Yet, they are lethal. Should the parents of teenage boys be called irresponsible for allowing their under 18s to ride motorbikes? I am genuinely interested. Lots of things that are legal are, in my opinion, at least as dangerous as things that are not legal.
Personally, I would never, ever let my teenagers go on a motorbike because I consider the risks are too high. (My parents forbade me to ride as a passenger on a motorbike, of course I ignored them and in fact had a preference for boyfriends with motorbikes. I just walked a few hundred yards up the road and got on the motor bike where they couldn't see me.) I guess I got a thrill from the danger aspect and disobeying my parents. I had a few close misses, but never a bad accident. Lucky, I guess. I know so many people who have had really bad accidents on motorbikes (and a few who have died).
I would probably be quite happy about quad bikes (off road) even with much younger kids, because I wouldn't think the risks were so high. We thought about doing it in Egypt when the kids were quite young, but didn't get round to it. I think it probably was a little dangerous - but not as dangerous as driving on the M25 on a bad day.
Now, what about children on bikes on roads? This is perfectly legal but also (in busy urban areas) most definitely at the higher level of risk. I used to feel quite hysterical when my husband took our small children on child seats on the back of his bike. Perfectly legal - but in my opinion unacceptably risky on a road (not off road though).
I guess we all have to make our own risk assessments. Life is inherently risky - a friend of my 16 year old's was recently run over and killed - she walked in front of a bus, just didn't look. It's the things we do every day without really thinking about them - like driving, crossing the road, that are the most risky. Any time any of us step on or near a road, as a pedestrian, driver, cyclist whatever, we are taking a risk.
I think in the case of this child on a quad bike, they were very unlucky. It was a quiet country road and they were probably lulled into a false sense of security as it was near their home and they felt 'safe' there, almost as safe as going across a field or something. Quite young kids ride ponies on quiet country roads - it's legal but not necessarily particularly "safe" ditto bikes.