An estimated 5,000 children die daily from diarrhoea, but half of those deaths could be prevented if they washed their hands with soap before meals and after going to the toilet, UNICEF spokeswoman Veronique Taveau said.
To raise awareness on the issue, the UN children's fund is launching the first Global Hand-Washing Day on Wednesday.
The day opens up a "real global battle for hygiene, in which the protagonists would be children themselves," said Taveau.
"The children are also those who, once at home among their family members and community, will introduce... a simple gesture that saves lives," she said.
In over 60 developing countries, the vast majority of them in Asia and Africa, activities would be launched in schools to introduce children to the importance of hygiene and hand-washing in particular.
Among Asian countries, hygiene conditions are particularly bad in India, Bangladesh and Afghanistan. In West Africa, diarrhoea is the third cause of mortality among children.
"Hand-washing with soap is the most efficient intervention and the least costly way to prevent deaths by diarrhoea" as well as from cholera and pneumonia, UNICEF's Taveau said.