More than 300 child soldiers were released by armed groups in South Sudan, the second-largest such release since civil war began five years ago.
Over 19,000 children are thought to have been recruited by all sides.
The "laying down of the guns" ceremony for 87 girls and 224 boys was the first step in a process that should see at least 700 child soldiers freed in the coming weeks, the United Nations said.
Putting down weapons and rejoining normal life is just the "beginning of the journey," said the head of the UN mission in South Sudan, David Shearer.
The UN has released almost 2,000 child soldiers so far. More than 10 percent of them have been under age 13.
Human rights groups say child recruitment continues, even as South Sudan's government says it has committed to ending the practice.
"The continued recruitment and use of children by the military and opposing armed groups points to the utter impunity that reigns in South Sudan, and the terrible cost of this war on children," Mausi Segun, Africa director at Human Rights Watch, said in a new report this week.
The released children will be reunified with their families and given three months' worth of food assistance and psychosocial support, along with the opportunity to go to school.
Source: News Agencies, Edited by website team