FARC rebels freed 13 minors from their ranks as part of a deal that could see the end of the 52-year civil war that has claimed 220,000 lives, according to the International Committee of the Red Cross, which received the youths.
The ICRC already turned over eight of the children to UNICEF Colombia, which said they were in good health.
"The girls and boys were received in establishments suitable for the process of reestablishment of their rights," the organization said, "so that they are able to develop in the best conditions possible."
The ICRC said the five remaining minors will also be handed over to UNICEF Colombia.
Neither UNICEF nor the ICRC identified the children, saying that "discretion is key to the success of such missions," which are expected to continue under a treaty between FARC and the Colombian government.
For decades, the rebels have forced children to become soldiers, training them as guerrillas to lay mines and fight.
In some cases, minors have been kidnapped. But other minors, particularly those who live in remote, marginalized and impoverished areas of Colombia, have been lured into the group by the prospect of food and shelter, according to Natalie Springer, author of a 2012 study called "Like Lambs Among Wolves.
Historic peace deal in the cards?
The children's release follows Colombian President Juan Manuel Santos' announcement last month
that he will sign a historic peace deal with the rebels on September 26 in the colonial city of Cartagena.
The signing comes ahead of an October 2 referendum vote that will allow the nation to decide whether to accept the agreement reached with the FARC after almost four years of negotiations in Havana, Cuba. FARC is a Spanish acronym for the Revolutionary Armed Forces of Colombia.
It's not the first time there's been an attempt at a peace deal, with previous truces between the groups falling apart in 1990 and 2002.
While hundreds of Colombians celebrated the most recent agreement
in the capital, Bogota, some agreed with opposition complaints that the Havana deal fell short.
Child soldiers do 'the dirty work'
FARC rebels have recruited tens of thousands of children to do "their dirty work" over the years, according to Springer's study
on the guerrillas.
"They're installing landmines, they're transporting explosives, they're kidnapping, they're involved in all of the activities that the adults are doing," Springer said in an interview four years ago.
Almost 70% of those captured are 14 or younger, with some as young as 8, she said.
Colombian woman Sara Morales said she was a young girl when the main guerrilla group forcibly recruited her.
"When I was only 11 years old, I was raped by FARC guerrillas, and for 11 years I was abused and exploited by them," Morales said in 2012.
"We were a group of 300 children, and only 12 of us were lucky enough to survive."
A brief history of the war
The devastating war began in 1964 after the success of the Cuban revolution, with the rebels wanting to use force to redistribute wealth.
In the decades since, the armed group has seized territory, attacked government forces and interfered with political life through high-profile kidnappings.
The rebels hijacked a commercial airliner to kidnap a senator in 2002, one of at least three passenger plane hijackings in the early part of this century.
Among the group's most notorious feats was the 2002 capture of presidential candidate Ingrid Betancourt
, who was held deep in the jungle for six years before a Colombian military operation rescued her.
FARC also turned to the drug trade, making millions from trafficking in cocaine. Both the United States and European Union designated it as a terrorist group.
The insurgency has displaced as many as 5 million people -- more than one out of every 10 Colombians.