The Obama administration has opened a broad campaign of enforcement against farmers who employ children and underpay workers, hiring hundreds of investigators and raising fines for labor and wage violators.

A flurry of fines and mounting public pressure on blueberry farmers is only the opening salvo, Labor Secretary Hilda L. Solis said in an interview. Ms. Solis, the daughter of an immigrant farm worker, said she was making enforcement of farm-labor rules a priority. At the same time, Congress is considering whether to rewrite the law that still allows 12-year-olds to work on farms during the summer with almost no limits.

The blueberry crop has been drawing workers to eastern North Carolina for decades, but as the harvest got under way in late May, growers stung by bad publicity and federal fines were scrambling to clean up their act, even going beyond the current law to keep all children off the fields. The growers were also ensuring that the workers, mainly Hispanic immigrants, would make at least the minimum wage of $7.25 an hour.

“I picked blueberries last year, and my 4-year-old brother tried to, but he got stuck in the mud,” said Miguel, a 12-year-old child of migrants. “The inspectors fined the farmers, and this year no kids are allowed.”

Child and rights advocates said they were encouraged by these signs of federal resolve, but they were also waiting to see how wide and lasting the changes would be. Across the country, hundreds of thousands of children under 18 toil each year, harvesting crops from apples to onions, according to a recent report by Human Rights Watch detailing hazards to their health and schooling and criticizing the Labor Department for past inaction.

“The news from North Carolina shows the value of strong enforcement,” said Zama Coursen-Neff, a lawyer with Human Rights Watch and the report’s author. “We also need to change the law to make sure this isn’t a flash in the pan.”