Sub-Saharan Africa will be the main focus of the campaign
It also says that every minute, a child is infected with HIV and another child dies from an Aids-related illness.
The charity says children are being overlooked in the global fight against HIV and Aids.
Unicef says only 5% of HIV-positive children get medical help, and fewer than 10% of the 15 million already orphaned by Aids get financial support.
Unicef's executive director Ann Veneman said children were the "invisible face" of a very visible disease and were missing out on the help that adults received.
"It is critical that the world unite for children and unite against Aids. The size of the problem is staggering, but the scale of the response has been inadequate," she said.
'Years of neglect'
The children's charity is launching a global campaign, Unite For Children, Unite Against Aids, which will focus on Africa, but also target Central Asia and Eastern Europe.
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The campaign will focus on preventing mother-to-child transmission, educating young people on how to protect themselves, and on making sure HIV-positive children get the medical treatment they need.
Although sub-Saharan Africa is the focus, Unicef says 80% of all those living with HIV in Eastern Europe and Central Asia are under 30, and that rates of infection are climbing faster there than anywhere else in the world.
The BBC's East Africa correspondent, Karen Allen, says Unicef is using Kenya to highlight what it says are years of neglect of youngsters either infected or affected by Aids.
The time for the blame game is over. We are asking for collective action... from governments downwards
Naisiadet Mason
Unicef adviser
Orphans bear cost of Aids
She says the campaign is calling for cash subsidies to pay for school fees, food, and shelter, transforming the lives of youngsters who might otherwise be forced into prostitution or crime.
Nairobi-based Unicef adviser, Naisiadet Mason, an HIV-infected mother of two, said the campaign was long overdue.
"The time for the blame game is over. We are asking for collective action... from governments downwards," she told the Reuters news agency.
"The day I was told I was HIV positive I was devastated and alone. We must create the networks, remove the stigma, supply the drugs," she said.
"We want to get HIV treated as just another disease because we know it is manageable."
As UNICEF launches a global campaign to help the youngest victims of HIV/AIDS, the charity has warned that 18 million children may be orphaned by the disease by 2010.

They say that every minute, a child dies from an AIDS-related disease, a child becomes infected with HIV, and four people in age group 15-24 become infected with the disease.

UN Secretary General Kofi Annan said: “Nearly 25 years into the pandemic, help is reaching less than 10 percent of the children affected by HIV/AIDS, leaving too many children to grow up alone, grow up too fast or not grow up at all. Simply put, AIDS is wreaking havoc on childhood.”

The campaign, "Unite for Children, Unite Against AIDS" will be launched this afternoon in New York. Along with goodwill ambassador Jemima Khan, and UNICEF's special ambassador and former Bond actor Roger Moore, several world leaders and governmental ministers from around the world will be attending the event.

The UNICEF advisor in Nairobi, Naisiadet Mason, told Reuters: "The day I was told I was HIV positive I was devastated and alone.

"We must create the networks, remove the stigma, supply the drugs. We want to get HIV treated as just another disease because we know it is manageable."

UNICEF will be using Kenya as a case to highlight the "years of neglect" of youngsters infected or affected by HIV/AIDS.