After another spring break spent lugging 50-pound buckets of vegetables and using sharp shears to cut onions, Norma Flores López returned to school with her hands too swollen to hold her pencil. She fell behind in her schoolwork. It wasn't the first time.

From age 12, López, who is now 26, worked 12-hour days, seven days a week on farms during peak season. Children must be at least 16 years old to work in most industries in the United States, and generally do so with strict limitations on the number of hours they work. For hazardous work such as manufacturing and mining, the minimum age is 18.