COTONOU (Reuters) - The scars splitting the palms of Isidore's small hands remind him of the six months he spent crushing stones in a Nigerian quarry.

The 13-year-old from the tiny country of Benin is one of tens of thousands of children sold into near-slavery across West Africa each year. The problem is especially acute in dirt-poor Benin, best known as the ancestral home of the voodoo cult.

This month, Isidore was rescued along with 73 other boys, some as young as four. Looking tired and thin, the children were handed over to Beninese officials at Krake on the 700 km-long (430 mile-long) border with oil-rich Nigeria.

It was the second time in a month that Beninese children had been repatriated from Nigeria under an agreement to stop cross-border crime. In September, 116 children were sent home after they were found working in quarries.

The rescues offer rare hope in a region where childhood is a luxury many can't afford. But they also illuminate a dark trade fuelled by greed and shored up by tradition and necessity.

Isidore says he was taken from his home in the town of Za-kpota in central Benin by an uncle. He ended up breaking stones near Abeokuta in southern Nigeria.

"I only did six months. I didn't get anything because the police came to get us," he said at a sports stadium in Benin's main city Cotonou where the rescued children were being cared for by UNICEF after their repatriation.

Dressed in cut-off jeans and a T-shirt, Isidore sat on a bed in one of three ad-hoc dormitories, eagerly gulping down his first hot meal in a long time -- rice in tomato sauce.

WHIPPED NOT PAID

Benin has long been a source of cheap labour. Several hundred years ago it was dubbed the "Slave Coast" by the European powers who set up trading posts there to ship tens of thousands of slaves to the Americas, and Haiti in particular.

Now it provides illegal child labour for richer countries in the surrounding region, such as Ivory Coast, Gabon, Congo, Cameroon, Nigeria and Equatorial Guinea.

Philippe Duamelle, UNICEF representative in Benin, says children can be taken away by members of the extended African family -- as in Isidore's case -- or by professional traffickers from both sides of the border with Nigeria.

"The children work in unacceptable conditions. Their parents are paid derisory sums of money," he said.

Traffickers sometimes pay as little as 20,000 CFA francs ($35) per child. The money is sometimes supplemented with cheap gifts such as lanterns, bolts of cloth or bottles of gin.

"Some say they will place the children with families who will take care of their education. Others admit they will work but say they will live in good conditions and be able to build a better future for themselves," Duamelle said.

Some parents are duped. Others know the reality of the deal -- but poverty often leaves little room for compassion.

The dealers can sell the children for up to 250,000 CFA francs ($440).

Duamelle and his team visited Za-kpota this year and found a well-organised system, where children were taken away for three years at a time then given a break. After nine years, or three cycles, they were freed. Some later became traffickers.

Maxime, an 11-year-old with weeping wounds on his knees, was rescued with Isidore and is from Za-kpota. He worked at the quarry for a year but his boss kept his salary.

"You have to have your knee on the ground to work well," he said, explaining his wounds. "We had to be good to avoid the whip. I was whipped twice at the beginning."

UNICEF says some 200,000 children are believed to be trafficked each year in West Africa. They work on cocoa and coffee plantations in Ivory Coast and in diamond mines in Sierra Leone. Many become domestic servants.

Officials at Benin's Minors Brigade say 326 children have been repatriated or intercepted so far this year. UNICEF quotes Nigerian sources as saying thousands of Beninese children might still be exploited in Nigeria.

Non-governmental organisations in Benin are trying to persuade parents to keep their children at home. Local activists alert police when traffickers are spotted. But Duamelle admits much more needs to be done.

"We need to redouble our efforts and combine the energies of everyone -- governments, non-governmental organisations, the private sector and the media," he said.

Ending the practice will be particularly difficult because of a tradition known in Benin's Fon language as vidomegon, under which children from rural families are often placed with relatives in cities to work as domestic servants.

"There has been a perverse mutation of the system and this leads to trafficking," Duamelle said.