Actually, a kid who worked hard could make decent money, for an unskilled worker. The threat of being sent to work in the fields also inspired many a more enterprising youth to find alternative employment.
My hardest manual labor job, between years of college, was in an adobe brickyard. It was not unusual to move ten tons of bricks in a day, though that usually meant about 4 or so hours of work, after which one was spent. Stacking bricks, it was common to have fingertips mashed between bricks, which was remarkably painful. The back, of course, was constantly fatigued. While I wouldn't wish it on anyone as a career, it was a valuable learning experience many would benefit from.