Photos That Helped End Child Labor in the U.S.
motherjones.com
motherjones.com
And toward the end of the article is a photo [1] of Shorpy Higginbotham, whose name was used for an interesting historical photos site [2]. There are three photos of Shorpy in existence [3], staring back at us from 100+ years ago, and a few details of his short life.
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