Now, all grown up, Llanos is back at the jail -- but this time he's in charge: a graduate with a law degree, serving as the administrative head of the country's prison system.
But for many other Bolivians who grew up in prisons, the ending did not turn out so well. Traumatized by the experience, many later fell into lives of crime and are now back behind bars, this time serving as convicts.
"Forty years ago, this place was so much quieter," Llanos recalled, pointing at the packed courtyard of the San Pedro prison, in the heart of La Paz.
Today the overcrowded jail overflows with inmates, who are forced to share cells or find spots in hallways and attics.
The children -- 160 of them living in this detention center with their fathers -- may not jump out at first glance, but after a while, you start to notice them.
Just over there, a little girl is playing, looking cute and sporty in pink, and seemingly without any sense of where she is.
Bolivian law allows children up to six years old to live in prisons, if custody is granted by a judge to the incarcerated parent.
"There's not even one case where that has happened," Llanos noted, but nevertheless, the children are here, and the police turn a blind eye.
Some 1,600 sons and daughters -- from infants to teenagers -- live in Bolivian prisons with their parents. And unlike in the women's jails, men's prisons like San Pedro have no facilities for the kids' schooling and care.
It may not be ideal, but the kids "don't have anyone else to stay with," argued Richard Hernandez, serving time for drug dealing, and a spokesman for the fathers of San Pedro prison.
"Some are orphans, or more often, the mother has also been detained," said the father, whose eight-year-old lives with him in jail.
Others say the kids are safer in jail with their parents than they would be outside, where even more dangerous conditions may exist.
But within the prison walls, the children live among thieves, murderers, rapists, gang members and drug dealers -- and are witness to drinking, drugs and violence.
Some parents, alarmed by the dangers all around, don't let their children leave their cells, creating an even more confined existence.
Either way, the children "live with continuous psychological pressure" in an aggressive environment that is really not meant for children, said Stefano Toricini, a volunteer from an Italian aid group that has for a decade provided educational support to minors in San Pedro.
"It is traumatic to live in such a place," he emphasized.
The damage is long-lasting: children learn how to relate to others based on the prison code, where the strongest rule by force and where everything has a price.
"Society should be more worried about this," said Llanos.
"We have to do something to deal with this now, because if not, that child who grew up in jail is going to kill you later," he predicted.
In theory, the young children are protected inside the prisons, but no one really knows what happens in the tangle of dark prison corridors.
Children turn up beaten and sexually assaulted, and authorities only respond when something "somewhat serious" happens," said a criminal psychologist who requested anonymity.
Even then, while the attacker may be punished in solitary confinement for a while, the child has no escape and must continue to live where they got hurt.
"Children have no voice. Some scream, but the majority don't," said Toricini, the Italian volunteer.
Llanos announced at the end of the year that San Pedro prison will build a space above the chapel for the kids, serving as a bedroom, school and recreation center for the children.
But for now, kids six and over attend a nearby public school in the evenings -- except for those who go out on the streets looking to make a living.
The prison kids -- around 130 -- speak and act more aggressively than the other students, said Reynaldo Pacheco, the director of the Gran Bretana school. And their schoolmates often tease them for being "inmates kids," he added.
Despite the problems for the kids in jail, it is not clear what the solution should be.
Toricini leaned towards keeping the kids out, but a spokesman for the jailed fathers argued that the kids are the only contact the prisoners have with the outside.
Forbidding the children to stay could lead to a riot, authorities warn.
It is a sensitive issue that "must be treated with kid gloves," they add.