Yeah, I think that when it comes to young children, you err on the side of caution. I've been racing motorcycles (motocross, hare scrambles, road racing) since the early 1970's, I've been riding ATVs since the early 3-wheeler days. I've seen too many bad things happen too quickly. Having dad around watching the little guy ride his 350 might only mean that he watches his child die. If he's 50 feet away and the kid gets into trouble, what's he going to do? Realistically, how is he going to stop something from happening when the kid is on an ATV that gets away from him? Year before last, there was a girl just a couple of years older than this boy at an ATV park in Georgia (near my house) that was killed in an ATV crash....Daddy was right behind her. What if the ATV starts to roll? Does the kid have enough body weight to keep in on it's wheels? Rollover crashes are some of the most dangerous crashes....if you are 8 years old and you have a 600 lb machine rolling over you, you stand a good chance of being seriously injured or killed. You started out at his age on a 350....you didn't get hurt while you were learning. That's great, but you were lucky. That does not in any way mean it's OK for everybody and it's going to turn out as well for everyone as it did for you. It could have just as easily turned out badly for you. There are rules governing the size of the ATV that children should ride. Those rules are in place for a reason. Most kids would probably end up just fine starting out on bigger machines....but a percentage of them wouldn't. A much higher percentage would be injured riding machines that were too big than would be injured starting on a properly sized machine. You were a part of the majority of kids that don't get injured. But, would you really take a bigger than necessary chance on your kid being one of the unlucky ones? Riding ATVs and motorcycles carries enough inherent risk when done properly...there's no need in unnecessarily increasing that risk.
It's just like people that allow their kids to ride ATVs without helmets, chest protectors, gloves, boots, etc. It's irresponsible and dangerous. Why risk your child's safety when it's so easy to enjoy the sport in an acceptably safe manner?