UN Special Envoy for Children in Conflict Olara Otunnu, who has been visiting Afghanistan, said that the country's ill-equipped schools, lack of food and poverty meant a dismal existence for many young Afghans.
He said one in four Afghan children die before they reach the age of five, and half of all young children suffer from chronic malnutrition due to an inadequate diet of black tea and bread, which some children are forced to exist on for weeks at a time.
Such inadequate nutrition can lead to medical problems and deformations in later life.
"It leads also to a large number of kids being on the streets working because they have to earn bread for their family," he told the Associated Press news agency.
Commitment needed
Mr Otunnu also condemned conditions at refugee camps he visited near the southern Afghan town of Spin Boldak as "absolutely deplorable".
Around two million Afghan children are thought to be refugees.
"[My visit] confirmed just how extensively damaging the two decades of war have been for the children and youth of Afghanistan," he said.
"All displaced people long to return home but, for children, transitions can be difficult."
Mr Otunnu praised the commitment of international aid charities in the country but said that only long-term funding and firm commitment from the international community would provide solutions.