In Iowa, it is legal for (US citizen) children as young as the age of 12 to work on farms, for an unlimited number of hours, so long as their parents grant permission and they do not miss school. They have so far been unsuccessful at legislating to raise the age limit from 12 to 14[0] as the Children's Act for Responsible Employment act has failed to pass, due to strong opposition from agricultural industry groups.

Many of these children are pressured into it by parents who have no other financial options, as described in the video I linked above.

[0] https://www.iowapublicradio.org/news-from-npr/news-from-npr/...