Children at the border.

"They were barely old enough to cross a street by themselves, much less a border," wrote The New York Times last month. The Times was referring to two Mexican boys, 6 and 8, whom U.S. agents had seized from smugglers at the U.S.-Mexican border.

Each year, border officials detain thousands of people who try to enter the U.S. illegally. Now, a growing number of detainees are children unaccompanied by their parents. During the first nine months of this year, Mexican authorities stopped more than 9,800 children traveling without their families.

Many of the children were seeking to join parents who are illegal residents in the U.S. For years, illegal immigrants from Mexico and Central America have made the painful decision to leave their families at home to seek work and a better life in the U.S. Many found a way to go home periodically, or waited for their families to follow.

But a decade ago, concern over the rising number of people trying to enter the country illegally caused U.S. lawmakers to call for stricter border controls. After 9/11, security was further tightened. This has made it much more difficult for illegal immigrants to return home to see their families. Instead, many pay as much as $7,000 to smugglers, called coyotes, to bring their children to the U.S.

Most coyotes are women who pose as a relative of the child they are attempting to smuggle into the U.S. Other coyotes try to avoid law-enforcement officials by undertaking an even more dangerous journey: They cross a remote desert between California, Arizona, and the Mexican state of Sonora. According to authorities, an illegal immigrant died in the desert each day last summer.

For children who are caught, a terrifying journey usually ends in their being sent back home. But often, their parents simply try to smuggle them into the U.S. again.

"If my children stay in El Salvador, I will definitely lose them because of the distance that separates us," said one Salvadoran man who works illegally in Washington, D.C. "If they come with a coyote to the United States, there is a chance that I will lose them in the desert.... [But] if we want to be with our children, there is no other way."