Hundreds of thousand? I think that is a profound exaggeration, according the the BLS there are ~750,000 agricultural jobs in the US. Also many children working on farms are children working on their family farms.
Take a look at this link, I didn't make it up. TAKE ACTION: End Child Labor in US Agriculture | Human Rights Watch
Children between 12~14 can only legally work on farms if their parents also work on the farm , can only work during non-school hours, can't work anywhere near 14 hours a day and aren't allowed to do hazardous agricultural work. Here is the youth employment guide for agricultural employers:
http://www.dol.gov/whd/AG/ag_pocket_guide.pdf
Banning children on farms would actually hurt small local farms as many of them are family farms and have their children working on the farm.
Just because there is a law regulating child labor doesn't mean anyone follows it. We are talking about money and greed.
The same government also told us that Iraq had WMDs. Trillions of dollars changed hands and thousands of lives were lost. No WMDs found. Thats why groups like Human Rights Watch are vital.
And you know full well I wasn't talking about small family farms where the kids of the owners work on the farm, i.e. chores.