"Today we have had a magical day," Petro told the media upon his return from Cuba, where he signed a six-month truce with Colombia's last active guerrilla group, the ELN.
"Getting closer and attaining peace in the agreement that is moving forward with the ELN... And now I return and the first news is that indeed the Indigenous communities that were in the search and the military forces found the children 40 days later," he said.
"They were alone, they made it on their own. An example of absolute survival that will go down in history," he said.
Seventeen days after the children went missing, Petro announced that they had been found alive, but he retracted the statement a day later, saying he had been given false information.
On Friday, he praised "the effective coordination between the military and the Indigenous people" during the search, saying it was an "example of an alliance for the country to follow."
Valencia told AFP that the children had been found by a native of Araracuara who had been participating in the search.
"I need a flight or a helicopter to go and get them urgently," the grandfather said.
Defense Minister Ivan Velasquez paid tribute to the various army units' "unshakeable and tireless" work, as well as to the Indigenous people who took part in the search.