You're right, kids used to do these jobs. I grew up in a rural community. As a teenager, we knew the farming seasons that would require lots of labor: putting out the tobacco crops, hay season, harvesting the tobacco crops. Farmers would stagger their jobs because they knew they needed us teenage boys to do the work. Yes, undocumenteds do those jobs today but it's because teens won't work like we did. My dad still farms. When he has citizen kids/teens/ 20 somethings come out to work, they often work one day; demand their pay at the end of they day then usually don't show up the next day. I was there one day when he brought in a new twenty-something. You could tell by looking he was a "user". He farted around for several minutes while his Mexican colleagues worked. He had to keep tabs on his cell phone (probably a free govt phone). I don't know if he finished the day.
Farmers would gladly use teens that were willing to work (like we did). Unfortunately, our kids today are not up the to the task when it comes to demanding physical labor.