Advocates who study migrant farm worker families estimate that as many as a half million of children currently work in the fields of America picking our food.

And some of them are doing more than that. First NBC Bay Area found dozens of children who work and have worked the fields of California picking, trimming and cultivating fresh fruit and vegetables that end up on American consumers’ tables.

But the Investigative Unit also found children doing even more dangerous work.

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The Unit traveled across the country and found children working the tobacco fields of eastern North Carolina. Advocates tell NBC Bay Area’s Investigative Unit that they have pulled children as young as 9 years old out of the tobacco fields in North Carolina and found numerous 10 year olds in those fields.

In the fields of Eastern North Carolina undercover cameras found workers topping tobacco.

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It is a difficult skill that must be taught and learned under the hot summer sun in the Carolina Piedmont. Workers must learn to quickly and efficiently take the flower off the top of the tobacco plants in order to increase the yield and quality of the remaining tobacco leaves. And most workers do it with gloves and long sleeves.

But not one girl we found who told us at first she was 14 years old. Like all of the young workers NBC Bay Area followed both in North Carolina and California during our investigation we are not revealing this teenager’s real name or the exact location where she works to prevent retaliation.

"I’m the youngest," the girl tells us. Later, she changes her story about her age revealing that she’s actually 13 years old. She also acknowledges that that is too young to be out in the tobacco fields and if she were caught she might be kicked out of the field.

"I just need to cover my face," the girl says. "I have to hide it (with a scarf or hat.)" Another teen said she started working in the eastern North Carolina tobacco fields when she was twelve with her mother’s encouragement. We will call her Maria. “I was 12 so I was like the youngest one there,” Maria said.

Maria described how her mother first prepared her to top tobacco. She does it, Maria said, to help out with the family’s finances. "She told me to like wear long t-shirts (and) take a hat,” said Maria. "(Mother told me) ‘Don’t yank the flower off” because I would cut myself like that. She’s like ‘just bend it like that and throw it away.’”

Because workers who top tobacco also are exposed nicotine from the plants advocates for children say young workers doing the topping are especially vulnerable. “We don’t allow these kids to smoke because we recognize these dangers,” said Norma Flores Lopez, director of the Children in the Fields Campaign of the Association of Farmworker Opportuntiy Programs. ”Especially the dangers of nicotine.

But we are allowing these kids to be exposed to those dangers as they are harvesting the Tabaco leaves and there is nothing being done about it. Flores Lopez has been following working families in agriculture all her life. She was once a migrant child farm worker too.

"We have kids that are working in the tobacco harvest,” said Flores Lopez, “where it is perfectly legal for a 12 year old to be working out there, harvesting tobacco leaves for a number of hours despite the fact that they are absorbing high levels of nicotine through their skin. Which would be equivalent to about 12 cigarettes a day.”

Those who supervise the work like this field boss say they don’t approve of children working in these fields. In fact one boss who we confronted at the field insisted that if children were present it was without his knowledge and that the children misrepresent their age.

In fact, some of the young workers admitted to us that they lie about their age and hide from the bosses in order to keep on working and keep on contributing to their family’s income.

We asked one field supervisor: “What’s the appropriate age for a person to work in the field? 18 years old?”

He agreed. “Yes. 18 years old. We don’t need little boys here.” But it’s hard to miss a five year-old in the fields or mistake him for an adult.

One 19 year-old native North Carolina worker said he’s worked alongside a five year-old boy around the tobacco fields near Kenansville, N.C. “That boy’s probably unstoppable,” said the worker.

“He (the five year old) is like a Mack truck at 5 years old.“ Our connections in North Carolina told us that after our TV crew left the fields there were death threats against some of the workers in the area. That’s why we will not reveal their exact locations and nor their identities.

NBC Bay Area’s Investigative Unit asked US Secretary of Labor Hilda Solis, who was in NBC Bay Area’s San Jose studios, if she would sit down and talk on camera about this issue. Solis’ staff declined our requests us to talk to her.