Across the Middle East country, nearly 10 million children -- 80 percent of the country's under-18 population -- need urgent humanitarian assistance, Xinhua news agency reported.
More than 1.4 million people have been forced to flee their homes in past six months of violence, the agency said in a statement released on Friday.
"With every day that passes, children see their hopes and dreams shattered," UNICEF representative in Yemen Julien Harneis said, adding "Their homes, schools and communities are being destroyed, and their lives are threatened by disease and malnutrition."
UNICEF said even before the conflict, the nutrition situation was dire as Yemen produces less than 10 percent of its food needs and relies heavily on imported foodstuffs.
The consequences for children are "dramatic", UNICEF said, adding the number of children under five at risk of severe acute malnutrition has tripled in 2015, with 537,000 children now at risk, compared to 160,000 children before the conflict.
The agency attributed the deterioration to food shortages and poor access to markets, reduced access to health facilities and sanitation, and the disruption of livelihood opportunities.
Scarcity of fuel, electricity, gas, water and other services is further exacerbating the situation.
The last six months have also seen a growing number of attacks on civilians and vital infrastructure, said UNICEF.
The UN is providing psychological support to help children cope with the horrors of the conflict, as well as education material on avoiding un-exploded ordnances and mines.
In March 2015, political crisis in Yemen rapidly escalated into all-out conflict. As the fighting has spread across the country, millions of civilians are suffering.
The crisis has been characterised by the use of explosives with wide-area effects in populated areas.