Children of Phnom Penh, David Barboza, The New York Times
Many children living in poverty are forced to work to support themselves and their families.
A large proportion of the world's 218 million child workers are in India, which came sixth in the poll.
"An estimated 60 to 115 million children are classified as working children - the highest number in the world," said Anuradha Mittal, director of the Oakland Institute think tank.
"Deprived of their childhoods, most have never seen the inside of a school."
U.N. Deputy High Commissioner for Refugees Wendy Chamberlin highlighted the case of Nepali girls who are trafficked to Indian cities, including Mumbai and Calcutta, for sex work. "They are really trapped," she said.
In Russia's breakaway Chechnya republic, fighting has displaced at least 95,000 people and UNICEF says 99 percent of residents live below the official Russian poverty line.
Child soldiers and forced labour were key reasons why respondents picked Myanmar, where the military junta is accused of conscripting tens of thousands of children to fight.
Egeland called on the international community to boost efforts to tackle children's issues around the world. "We must do more to achieve the Millennium Development Goals, particularly as they relate to children, who are, of course, our future," he said.
Some 1.8 million children have been affected by a three-year conflict in Darfur, according to the U.N. Children's Fund (UNICEF), where they risk being recruited to fight and are especially vulnerable to disease and malnutrition.
"It is a traumatised population and you can see it in the children's faces," said Hollywood actress and UNICEF goodwill ambassador Mia Farrow, who last month visited camps for some of the 2.5 million displaced by Darfur's war.
"Everyone has lost family, seen villages burn, seen relatives raped, been raped."
U.N. High Commissioner for Refugees Antonio Guterres - who selected Congo, Uganda and the Sudan/Chad border, where some 200,000 refugees from Darfur eke out an existence - pointed to the physical and psychological consequences of living in crowded, underfunded camps "which are not conducive for a healthy child development".
In southern Sudan, children also suffer the effects of low-level violence, poverty and a lack of basic services. The region is struggling to recover from a 21-year civil war with the north that killed 2 million people, as 600,000 refugees forced to flee the country trickle home.
AlertNet, a humanitarian news website run by Reuters Foundation, asked 112 aid experts and journalists to highlight the world's most dangerous places for children.
After Sudan, they chose northern Uganda, Democratic Republic of Congo, Iraq, Somalia, India, the Palestinian territories, Afghanistan, Chechnya and Myanmar - with the top three clearly ahead.
More than 2 million children worldwide have died as a direct result of armed conflict in the past decade, and about 20 million have been forced to flee their homes, according to UNICEF. More than a million have been orphaned or separated from their families.