Whatever the details of any accident, the real problem is the pervasive tendency in our society to push children towards maturity. Whether we are bragging that they can read at 3 or buying them toys (including ATV's) which are not suitable for their age, or exposing them to violent media which they are not equipped to handle, we are pushing them.
I use the term "we" loosely, of course. I know that most 'catters are not doing this.
Recently I picked up a book by a therapist who said, "In the 1950's, the big problem was the spoiled child. Now the problem is the pushed child."
I have noticed that in the comic strips, the kids are merely small-size adults now. We see 10-year olds going on dates and 7-year-olds criticizing President Bush. Obviously the cartoonists have decided that real children, with their childhood activities and concerns, are of no interest.
I'm not surprised that 6-year-old Chandler wanted to "be a man" and ride his ATV off on his own.