Unicef has launched an appeal to raise more than $800m (£450m) to help women and children affected by humanitarian emergencies in 29 countries.
Each year, 100,000 Sudanese under-fives die of preventable diseases
The organisation praised donors for their response to disasters such as the Asian tsunami and Pakistan earthquake.
But it said the number of humanitarian crises in 2005 had taken its toll in overstretched aid workers, under-funded relief projects and donor fatigue.
Most appeals got just a fraction of the money called for, the UN agency said.
Unicef's report, published in Geneva on Monday, said that despite breaking funding records in 2005, only four of Unicef's 25 emergency appeals had received 50% of the required amount.
Of the requested $805m, more than a third of the total is for Sudan, where continuing conflict in the western Darfur region is threatening the survival of 1.4 million children.
Dan Toole, Unicef's director of the office of emergency programmes, who launched the Humanitarian Action Report, said a swift response to crises was essential to meeting long-term development goals.
"In many of these countries, children live in an almost constant state of emergency because they are growing up in extreme poverty, without access to education or the most basic health services," he said.
The BBC's Imogen Foulkes in Geneva says Unicef is unlikely to be funded in full.
With partial funding, she adds, the organisation will be able to carry out life-saving work such as food distribution and immunisation, but other long-term health and education projects may miss out.
According to the 200-page report, areas in critical need of funding include:
- Sudan, where an estimated 17 million people are without access to safe water and more than 20 million are without safe sanitation
- Eastern and southern Africa, which have had more emergencies over the past decade than any other region in the world, including the current drought, the HIV/Aids pandemic and widespread malnutrition
- Central and Eastern Europe and the Commonwealth of Independent States, which includes several countries affected by low-level warfare and fragile ceasefires, while natural disasters pose a threat across the region
- South Asia, which has the highest rate of absolute child poverty, with 330 million children suffering from two or more forms of severe deprivation
- East Asia and the Pacific, which faced a major emergency following the tsunami, and is contending with other threats, notably confirmed human cases of bird flu
- The Middle East and North Africa, which are dominated by the situation in Iraq, and violence in the occupied Palestinian territory.