When I was in High School, I had to work in the fields during the summer picking strawberries, green beans, and digging flower bulbs. The kids were put in one field and the migrants in another. The migrants would be whole families. They were very hard workers, whereas we kids would be in a strawberry fight as soon as no one was looking. Picking strawberries is not fun. You push a little wheelbarrow between the rows in the mud on your knees. The migrants were all "wet backs", the term for illegal aliens at the time. We all knew it, it was just a fact of life back then. After the harvest, they would move on to harvest the pears, apples, melons, or whatever work they could find. Then they would go back to Mexico.
I am sure that is the way many of the illegal immigrants still work; they don't count the number of people who sneak back out of the country. The H-2a system is flawed. Harvesters do not work for just one employer; they work for many, and go to where and when the next harvest takes place.
I don't think you are going to find enough kids (nowadays, any) to fill the void.