Seasonal agricultural work like this has been done by kids for generations. There's a short time where you need a flexible workforce and then you don't need them the rest of the year. And our hours were totally variable based on the crop and the weather. Some weeks there just wasn't anything to pick and we'd have to wait. You couldn't count on this income as an adult trying to pay the bills or raise a family. There were adults working there of course but they had different jobs (like driving tractors) and more steady income. I don't think we were stealing jobs from adults.
In the end the number of kids willing to work in the fields went down and these days they don't even bother trying to hire locals. They started flying in seasonal workers from Puerto Rico and then Jamaica. You can save up enough money in a few months to go home and live off of for a while (plus any other seasonal work you can find).