In the slums of Naples, they work in cramped, badly lit ''laboratories,'' stirring glue for shoe factories, making artificial flowers, and polishing bodywork.
In the impoverished rural areas of India, where landless laborers earn less than $50 a year, their debt-ridden fathers are often forced to hire them out as ''bonded labor'' to pay off debts to rich landlords.
They arrive in Bangkok alone from the underdeveloped northeastern provinces of Thailand and are immediately drawn into prostitution, or work in thousands of tiny factories unregistered by the government.
They spend up to 10 hours a day hunched over looms in Morocco and Iran, making carpets and other handicrafts.
They are children at work - exploited, often unprotected by legislation, vulnerable, often exposed to health risks.
Abdelwahab Boudiba, a Tunisian lawyer who has prepared a report for a Human Rights Commission meeting here, warns that in developing countries, ''The combined effects of anemia, malnutrition, and overwork are often irreversible.'' His report finds calorie deficiencies in 58 developing countries.
According to the latest statistics from the Geneva-based International Labor Organization (ILO), 75 million children under the age of 15 are working today - 97.2 percent of them in the developing countries. Many here feel the real figure is almost certainly higher. Mr. Boudiba's report says it could be as high as 145 million.
The ILO is in a race against time to extend protection and to tighten existing legislation. No fewer than 22 ILO conventions or recommendations exist to regulate child labor.
The key convention, No. 138, came into force in 1973. It prohibits the employment of children under the age of 15 - except for ''light work'' that does not harm their health. So far it has been ratified by only 23 countries, including the Soviet Union but not the United States.
One reason, concede ILO officials, is that child labor is deeply engrained throughout the rural areas of the third world, where children perform essential tasks in the field. Here, formal schooling may not exist. Parents are often worn down by disease, and life expectancy is lower than it is in Western countries.
''Children working is not in itself a crime,'' agrees Leah Levin of the London-based Anti-Slavery Society, which has published several reports on child labor in recent years. ''Indeed, it may be essential, particularly with recession deepening.''
She points to an increase of children working after school in Britain, where unemployment recently topped 3 million. ''Sometimes they are the only ones in the family working,'' she said.
Officials in the UN, particularly the ILO and UNICEF, are trying to draw the line between ''exploitation'' and ''employment.'' All agree it is much easier to pinpoint the scandals than propose solutions. They also agree that although governments rarely encourage child exploitation, many of them gain handsomely from the cheap labor.
Abdelwahab Boudiba estimates that child workers would have received $21 billion in wages last year had they been paid at proper rates. Morocco, he points out, made $23 million from exports of crafts and carpets to the West.
''When a country is poor or in debt, it is not overconcerned about the price paid in the toil of children,'' he says.
Overshadowing the problem are the many facets of modern life in developing countries - the poverty of rural areas, the irresistible appeal of city life, education systems that encourage Western ambitions yet bear little relation to a job market or a country's needs, the breakup of the family.
A recent report from the ILO, meanwhile, warns that some 4 million children of migrant workers in West Europe risk becoming ''social misfits'' if more efforts are not made to ease their special linguistic and cultural problems.
More and more, UN officials are beginning to talk of combining short-term action - such as the tightening up of existing laws - with longer-term goals aimed at alleviating poverty and creating education systems that offer a more relevant syllabus and even combine schooling with work for children in rural areas of the third world.
But no one underestimates the difficulties - with government spending being cut back during the recession, Western aid drying up, and multilateral agencies being forced around to President Reagan's controversial view that free market forces offer the best solution to poverty.