Survival rates in the UK are relatively high
An estimated 250,000 children develop cancer each year worldwide.
A report from the Cancer Research Campaign and Imperial Cancer Research Fund says that 80% are either not diagnosed or denied potentially life-saving treatment.
The figures were released on Monday ahead of the first International Childhood Cancer Day, designed to educate the public about children's cancer and raise funds to help young cancer patients across the globe.
In Britain, where survival rates are particularly high, more than 70% of children with cancer are still alive five years after diagnosis.
But because so many countries lack adequate cancer care facilities, the world's overall survival rate is as low as 15%.
Cancer rates
The rate of children's cancer is roughly the same throughout the world, at between 75 and 140 cases per million children, but the pattern of incidence differs considerably.
In developed countries, the most common type of childhood cancer is leukaemia, accounting for about a third of all cases, followed by tumours of the brain and spinal cord, which make up around 20-25% of cancers.
Leukaemia and brain tumours tend to be less common in developing countries, where poorer hygiene means that lymphomas largely caused by infection with the Epstein Barr Virus (EBV) are often the most common kind of children's cancer.
Burkitt's lymphoma, associated with infection by a combination of EBV and malaria, accounts for half of all childhood cancers in some African countries.
International Childhood Cancer Day will highlight the need not only for wider access to existing treatments, but also for more research into cancers that mainly affect the world's poorest nations.
Thousands saved
Professor Vaskar Saha, an expert on childhood cancer from the Imperial Cancer Research Fund, told BBC News Online that survival rates could be improved in the developing world by relatively simply measures, such as training medical staff in diagnostic techniques.
He said: "People assume that treating cancer is complicated and takes a lot of resources, but actually a lot can be done where there is a will.
"For instance, a local training programme in El Salvador has led to survival rates rising to over 50%."
Geoff Thaxter, International Childhood Cancer Day Coordinator, said: "We are delighted to announce the launch of this event. The response and enthusiasm of parent organisations around the world has been tremendous."
The family of four-year-old Ashleigh Darnes, from Spalding in Lincolnshire, has also backed the project.
She was treated for neuroblastoma one of the most common cancers among young infants - at Queen's Medical Centre, Nottingham in 1999.
She's now back at school and her mother Clare said: "We feel incredibly lucky that Ashleigh received such good treatment. If she'd been born in a poorer country, she probably wouldn't be around now, which is why I feel it's so important to support International Childhood Cancer Day."