He has come to put children on the international agenda - a tall order in a country where 15 million live below the poverty line and a third of all paramilitary fighters are children. At a home for prostitutes and former gang members, Mr Otunnu meets Valeria, a 15-year-old "sicario", or assassin. Sicarios are often paid by the drug barons to kill at point- blank range. With no emotion, Valeria talks of how she held up a taxi driver and cut his throat "by accident". She coyly admits that in the future she wants to be a marine biologist.
TUESDAY: Next stop, Soacha, a shanty town on the outskirts of the capital, Bogota. The police refuse to patrol this area, and Mr Otunnu is accompanied by a security guard. At a school run by police cadets because of the murder rate among the teaching staff, he is told that this is a microcosm of Colombia's fractured society. Among the students are gangs loyal to the paramilitaries, the rebels and the drug barons. And the school building reflects the nation's recent history - battered and decaying. There are few books and virtually no furniture. Mr Otunnu hears that many children witnessed the recent murder of a teacher, but that not a single one would talk about it.
WEDNESDAY: Mr Otunnu meets Javier. He left his village at 14 and joined the rebels. He says he did it because he was hungry. He gave himself up to the army at 16 after seeing a group of his friends slaughtered for trying to desert the guerrilla militia. They were caught, tied to a tree and Javier saw a bullet go into each head. Mr Otunnu, who is from Uganda, sings an Acholi welcome song. This should be surreal, but somehow it isn't.
THURSDAY: Before leaving Colombia, he meets the other UN agencies on the ground - the children's fund, Unicef, is there, as is UNHCR - the refugee agency - and UNDP, the development programme. When he was appointed in 1997, critics said yet another layer of UN bureaucracy would be created. But perhaps he is pointing the way. With an office of fewer than 12 people, he has wrested pledges from the Tamil Tiger rebels in Sri Lanka not to recruit children under 17. And he has obtained commitments, from governments and guerrilla factions in Sudan, Burundi, Congo and Angola not to use soldiers under 18. Whether they fulfil their promises is another matter.
FRIDAY: New York. The contrasts in Mr Otunnu's life are huge. Today he is at UN headquarters drumming up support for a security council resolution on children - the first of its kind. He says that resolutions are international law, and that even rebel groups eventually want power and legitimacy. They may not have signed treaties such as the UN Convention on the Rights of the Child, but they can be shamed.
SATURDAY: Mr Otunnu relaxes. He has no children of his own. But he has adopted six from his brother and sister, who are both dead. If it were not for the bad blood between himself and the Ugandan President, Yoweri Museveni, he might have been secretary general of the UN. Museveni intervened to prevent it and Kofi Annan ultimately took the top job.
Olara Otunnu was interviewed for the Children in Arms documentary, to be broadcast on BBC Radio 4 at 8pm on 7 September.Reuse content