As well as the more than one million displaced by the Sudan conflict, at least 330 children have been killed and more than 1,900 others wounded, UNICEF said in a statement on Thursday.
"Many more are at grave risk."
Mandeep O'Brien, UNICEF representative in Sudan, said: "The future of Sudan is at stake, and we cannot accept the continued loss and suffering of its children."
"Children are trapped in an unrelenting nightmare, bearing the heaviest burden of a violent crisis they had no hand in creating - caught in the crossfire, injured, abused, displaced and subjected to disease and malnutrition," he added.
UNICEF said an estimated 13 million children were in "dire need" of humanitarian assistance.
The UN agency said it was ready to assist them but called for "safe, unrestricted access and guaranteed security to all areas where children are in desperate need."
It said the situation in the western region of Darfur - where 270,000 children are said to have been displaced by the fighting - was especially concerning.
"The situation in West and Central Darfur, in particular, is characterized by active fighting, severe insecurity and looting of humanitarian supplies and facilities," said UNICEF.
"The lack of safe water has left hundreds of thousands of children at risk of dehydration, diarrhea, and malnutrition," it added.
For several weeks, aid workers have warned Sudan's economy and healthcare system are at risk of collapse.
According to Sudan's doctors' union, three quarters of hospitals in combat zones are out of service.
The situation is expected to worsen with the approaching rainy season threatening to make parts of the country inaccessible and raising the risks of malaria, cholera and water-borne diseases.
The ongoing violence in Sudan has claimed more than 2,000 lives, according to the Armed Conflict Location and Event Data Project.
The International Organization for Migration says the fighting has driven 2.2 million people from their homes, including 528,000 who have fled to neighboring countries.
UNICEF: One Million+ Children Displaced By Sudan War
FILE: A worker from International Committee of the Red Cross (ICRC) carries children during the evacuation of children and caretakers from the Mygoma Orphanage in khartoum, in Wad Madani, Sudan, on June 7, 2023.
KHARTOUM — The conflict in Sudan has displaced more than one million children including 270,000 in the Darfur region, the United Nations children's agency said, warning many more were at "grave risk."