(BBC) – More releases will occur in the coming weeks, said the agency, which helped negotiate the children’s freedom.
The soldiers were recruited into an armed group which has now made peace with the government.
Other rebel militias have been locked in a civil war since 2013.
Fighting began after President Salva Kiir accused his deputy of trying to foment a coup, triggering a descent into nationwide violence and forcing about 1.5 million people from their homes.
According to Unicef, around 12,000 children have been forcibly recruited by armed groups in South Sudan over the past year.