Obviously it would be more accurate to know how many of these children were doing work. I would think it would be appropriate to say, about half of the children were likely doing some labor on the farms (8 and over doing some chores around the farm). This would place the rate of injury to be about 8666 per 550000 child farm workers. This also doesn't take into account sanctioned versus unsanctioned work, like the example of the kid who decides he's going to help out by running the combine to surprise his parents or whatever.
But this whole conversation is stupid. Who the fuck is the government to decide what is too dangerous? To tell you how to raise your kids, or how you should feed them, or whatever. As much as frothing liberals like to bitch and moan about huge corporate farms and distributors ruining small agriculture - what the fuck do you think cutting their available labor force is going to do? People have been working their kids on their property or to help them farm since agriculture began.
It is always just one little intrusion after another, and people ignore it and ignore it as small potatoes. Some day you may look around and not recognize the country you live in or remember what freedom was.