View more videos at: http://nbcbayarea.com.

(Editor's note: Names have been changed and locations have been withheld to protect the minors in this story.)

SAN FRANCISCO — Thousands of children, many too young to drive, are hard at work putting in long hours in brutal conditions to make sure the rest of us eat well — and cheaply.

Follow Open Channel on Twitter and Facebook

During weeks of investigation into the close-knit and tight-lipped community of migrant workers, NBC Bay Area found dozens of children working the fields in the San Joaquin and Sacramento valleys — some who started work at 11 , 10 and even 8 years of age. Advocates say the number nationwide may be as high as half a million.

While 8-year-old children can't work in an office or a fast-food restaurant, a 1938 law allows them to legally work in agriculture.

"Children can work at any age on a small farm with their parents' permission. It's absolutely legal for a small farmer to hire a 6-year-old to pick blueberries," said Zama Coursen-Neff of Human Rights Watch, who produced a 2010 report that found child labor prevalent in fields across the U.S.

Critics of U.S. labor law say it's a relentless cycle: Young workers drop out of school to follow their families and the crops for work. They work a full day in the fields picking, trimming and cultivating fresh fruits and vegetables. They often work nine to 10 hours a day in 100 degree-plus heat.

Then they remain stuck in the fields because they never finish high school.

Read this story on NBCBayArea.com

A typical day in one Central Valley migrant labor camp starts at 4:30 a.m.

Among those workers is a 15-year-old boy, "Ralph," who joins dozens of other young people heading to work. Some of them were told by direct supervisors to lie about their ages to get past the bosses in order to work.

During Ralph's work day, it reaches 106 degrees.

"We get kind of tired, and our arms hurt," said Ralph, who said he's in his second year working full time in the fields. "It is too hard to be in the fields."

U.S. labor law, which dates to 1938, allows children 12 years old — and depending on the circumstances, even younger — to legally work in agriculture.

There are many other children like Ralph.

"Like seven years, since I was 8 years old until now," one 15-year-old said, describing when he started in the fields.

Another of the young workers said, "I was in sixth grade. I was 11."

Yet another young girl described working so hard when she was 11 that her fingers bled.

"I had to carry a box, and I had cuts on my fingers," she said. "I came out really tired. It was really hot, and I didn't really like it, but it was worth it to go help my mom."

Certain crops are harder to pick for the children than others.

"Well, right now it's tomatoes," a teenager said. "It's the hardest thing I've done. I have to (work hard), bending over, standing up, carrying the buckets and throwing them."

Because of the hard work and long hours, some parents are trying to keep their kids away from the fields, even though their families need the money.

The mother of one young girl forces her to stay in school away from the fields.

"She says because it's a lot of work," said the girl, whom NBC News is calling Carmen. "She doesn't want me to go through what she goes through (in the fields). She says it's really painful, hard work. Every night I massage her back so that she can feel better in the morning."

Carmen vowed to go to college and get a higher-paying job so she can support her mother and get her out of the fields.

"I told her that when I get older I'm going to buy her a house and stop her from working," Carmen said.

Carmen isn't alone in her dreams. All of the children interviewed for this report said they hoped the money they earned would help them break out of the cycle and live a better life.

"Right now, I want to be an artist, like drawing," one teenager said.

Coursen-Neff of Human Rights Watch stressed that as unfortunate as it may seem, those long hours in the fields are perfectly legal.

"You have to realize that many children who are working in hazardous conditions in the United States are working absolutely legally because U.S. child labor law — which is pretty good — has a big gaping hole in it when it comes to agriculture," Coursen-Neff said.

"Children are working in American fields at far younger ages for far longer hours and in far more hazardous conditions than all other working children in America," Coursen-Neff said.

Watch US News videos on NBCNews.com

"A child can work again for hire at age 12 on any size farm. And at age 14, they can work for hire even without their parents' permission," she said.

In other words, "a child of any age can work unlimited hours outside of school in agriculture even though, in all other forms of work, the number of hours that they can work is limited to make sure that they can get an education and to make sure that they're not put at risk."

Coursen-Neff said her research shows that low wages for migrant workers throughout the industry means those families need more workers in the field to make ends meet. It becomes an economic necessity that continues for generations.

Another group that hopes to change the practice is the Association of Farmworker Opportunity Programs, based in Washington.

"These kids know that there is a necessity in their family to be able to make ends meet, to be able to put food on the table, and are out there in those fields trying to make that happen," said Norma Flores Lopez, director of the AFOP's Children in the Fields Campaign.

The Fair Labor Standards Act of 1938 created two separate classes of rules, Flores Lopez said:" There is a set of rules that covers children working in every other industry, and then there is a separate set of rules for kids working in agriculture."

It's hard to pin down how many children are out there, because there isn't a lot of data, Flores Lopez said, "but from our best estimates that we have been able to get, we know that there is anywhere from 400,000 children to up to as many as 500,000 kids."

Those kids, picking everything from grapes to almonds, all said they are laboring so long and so hard out of pure economics. The reality is that their parents simply can't make enough money working the fields without their children's help.

One 15-year-old worker's mother put it plainly.

"With just my husband's salary, it's not enough," she said in Spanish, speaking through an interpreter. "The two of them need to work in order to have anything and to keep up,"

The U.S. Labor Department tried to change the law this year to further restrict and even prohibit some children from working in fields, but it met opposition from growers.

"What they were proposing was a little too strong, a little too restrictive," said Pete Aiello, a second-generation grower in Gilroy, Calif.

"The current regulations as they are, I think, are good. I think they are sound. I think it's OK for kids that young to be working. (It depends) now on how many hours that they work."

Aiello and his family have owned and run Uesugi Farms Inc. for decades, growing chilies, pumpkins, Napa cabbage and other vegetables.

Uesugi employs 180 people on its direct payroll and 500 to 600 seasonal contract workers, mostly during the harvest season.

After other critics lodged similar complaints in Washington, the Labor Department withdrew the proposed rules in April. Critics also said the rules as drawn up by the Labor Department would have hurt family farms, although department officials dispute that.

Aiello acknowledged that some fellow growers look the other way and employ children who are 12 and younger.

"I know it does happen," Aiello said. "And that's unfortunate."

On July 24, the House passed a bill to prevent the Labor Department from trying to change the labor law regarding children in agriculture in the near future. Backers said the proposed Labor rules would hurt have family farms and 4-H clubs.

Only one representative, Lynn Woolsey, a Democrat from Sonoma and Marin counties in California, spoke out against the legislation. Similar legislation has been proposed and awaits action in the Senate.

What most sides can agree on is that this issue is largely unknown.

"I think Americans are largely clueless about the labor in general that supplies their food," Aeillo said. "And whether it's their age or their ethnicity or their legal status or any of the above, I think Americans are in the dark about what's going on."

NBC Bay Area sought comment from a dozen other large grower organizations, like the California Farm Bureau Federation and California Citrus Producers, as well as large food processors and producers. All declined to comment or didn't return messages.

NBC Bay Area also asked U.S. Labor Secretary Hilda Solis for an interview. Her staff declined.