Governments and aid agencies should do more to help children protect themselves against HIV, the children's organisation Plan International said.
Children orphaned by AIDS are increasingly vulnerable, says charity
Millions of children in the world's poorest countries are vulnerable to HIV infection because of exploitation and discrimination, its report warns.
An estimated 1,800 children become infected with HIV every day.
The report was backed by Unicef which said that many young people were not able to choose safe sexual behaviour.
The 'Circle of Hope' report is published two weeks before the this year's International Aids conference in Toronto.
It states that 2.3 million children under the age of 15 are living with HIV and many of them have no access to any form of care or treatment.
Malnutrition also places children at risk of HIV infection.
By 2010 it is estimated that 24m children could be orphaned by Aids, making them highly vulnerable to poverty and exploitation.
Plan has called for policies to include education of children and adolescents in school and in other environments to prevent the spread of Aids, prevention of parent-to-child transmission, and the development of strategies to support vulnerable children, including orphans.
Poverty, gender inequality, lack of access to education and social relationships all contribute to the vulnerability of young people to HIV infection, the report says.
In the West African Republic of Benin, girls are frequently harassed on their way to, from, and even in school by predatory males.
The girls are often pressured into having unprotected sex. Many more are so desperate and hungry that they are forced to sell sex to older men for as little as the price of a meal a day.
"When the only choice you have is the chance you might catch Aids and die in a few years' time, or the certainty of starving to death in a few weeks, there is no choice," said Plan International chief executive Tom Miller.
"There are millions of sexually exploited children throughout the world; girls whose older husbands are infected, or boys who are under intense social pressure to practice unsafe sexual behaviour.
"If you are not able to practice safe behaviour or ignore the risks, it doesn't matter how much knowledge of HIV infection you have," he added.
'Education the key'
A spokesperson for Unicef said: "Despite some progress, children are still the missing face of Aids in the global response.
"More must be done to also reach the most vulnerable groups. Girls who are at risk of being trafficked are among those. They are at particular risk of HIV infection and often don't have powers to negotiate safer sex.
"Education is key in the fight against Aids. Over 113m children are still not going to school - two-thirds are girls.
"Beside all other benefits of education, it also equips children with better negotiating skills, particularly important for girls.
"Over 50 million children have already lost their parents in sub-Saharan Africa - their first line of defence.
"Many of these children will struggle and will be more likely to be forced into child labour or the commercial sex trade."
Plan said it wanted policymakers to ensure universal birth registration for all children orphaned by or living with Aids, protection of inheritance and property rights for Aids orphans and basic access to health care.
Deborah Jack, chief executive of the National Aids Trust said: "The international community must as a matter of urgency address the two evils which are making children so vulnerable to HIV - poverty and the denial of human rights.
"UNAids reports that fewer than 50% of young people globally have comprehensive knowledge of HIV.
"But information alone is not enough. We must ensure that all governments are acting vigorously to end gender inequality and to stamp out the sexual exploitation of children through both legal and economic interventions."