The incident comes as the administration of President Gustavo Petro steps up efforts to regain control of rural areas in Guaviare province.
In a statement, ombudswoman Iris Marin said the minors killed in the strike against the FARC-EMC rebel group had been forcibly recruited and were being used as "human shields".
Marin called on the Colombian government and rebel groups in the country to respect international humanitarian law.
The public defender had initially said that six minors were killed in the strike, but updated the death toll following a statement from Colombia's Forensic Medicine Institute.
"No child or teenager who has been recruited should be affected by military operations," Marin said.
"The armed forces must adopt precautions to protect children who have been forced to take part in hostilities."
Earlier this week, Colombian Defence Minister Pedro Sanchez blamed criminal groups for putting children in danger and told journalists that in his view, "whoever gets involved in hostilities loses protection, without distinction".
The deaths of children in military strikes are a sensitive topic in Colombia, where a former defence minister resigned in 2019, after it was found that the government covered up the deaths of eight children during an air strike in the province of Caqueta.
According to Colombia's military, at least 19 fighters were killed in the strike that took place on Tuesday in Guaviare.
Petro said Saturday that he had decided to order the air strike because the column of rebel fighters was advancing toward a position from where they could ambush a smaller group of Colombian troops.
"The death of any person is regrettable, and especially that of minors," Petro wrote on X.
"I took a risk to save the lives" of soldiers.
Petro suspended air strikes against criminal groups shortly after he came into office three years ago, to decrease the possibility of killing minors.
The left-wing leader had accused previous governments of committing "war crimes" when it was found that minors died in air strikes against rebel groups.
But air strikes against Colombian rebels resumed last year, as the Petro administration struggles to contain the expansion of groups that are fighting to take over territory abandoned by the FARC, the guerrilla group that made peace with the Colombian government in 2016.
Petro is an outspoken critic of President Donald Trump's strikes on suspected drug traffickers in the Caribbean, which he has described as "extrajudicial executions".
On Friday, he argued that his government's air strikes against rebel groups are different.
"Those who are falling in the bombardments of Colombian forces have machine guns, explosives, and have declared themselves members of an armed group," Petro wrote on X.
"They are trying to eliminate government forces and civilians with their lethal weapons."
16°C