If it helped to end child labour, then why child labour is still present?
http://www.bbc.com/news/magazine-27350413
https://www.hrw.org/news/2014/09/17/child-laborers-america-2...
I assume these exceptions exist because of an historical limited labor pool during critical time periods for farmers, such as harvest. For example detasseling corn is a common summer job for teenagers in the Midwest. https://en.wikipedia.org/wiki/Detasseling
Here are more details on the rules: http://webapps.dol.gov/elaws/whd/flsa/docs/hours.asp