The abduction occurred near Malakal, where thousands of people
have taken refuge following months of violence in the nation.
Kidnappers gathered around a community and conducted house-to-house searches, according to the U.N. children's agency. It said the victims included boys over age 12, who were taken away by force.
U.N. officials warned the abductors that they're violating international law.
"The recruitment and use of children by armed forces destroys families and communities," said Jonathan Veitch, the UNICEF representative in South Sudan.
"Children are exposed to incomprehensible levels of violence, they lose their families and their chance to go to school."
South Sudan has been embroiled in conflict since December 2013, when President Salva Kiir accused his fired deputy, Riek Machar, of trying to oust him through a coup.
Since then, militia loyal to both have battled each others' forces. Violence has quickly spread, with reports of mass killings and starvation nationwide.
World's youngest nation
Talks and repeated pleas for peace have yielded no results.
More than 1.5 million people have been displaced, according to the U.N., and thousands killed. Some civilians have fled to U.N. bases in the country, making the facilities targets for armed militants.
South Sudan seceded from Sudan in 2011 after decades of war, making it the world's youngest nation.
Since the split, the nation has battled various setbacks, including violence divided along tribal lines -- the Nuer community backs rebel leader Machar while the President is from the Dinka tribe.
The warring sides have signed ceasefire deals, but the bloodshed continues.