Six in 10 of those forced to flee their homes in the conflict-ravaged Kasai region - about 850,000 - are children, leaving them prey to attack, detention, sexual violence, and recruitment by militia fighters, according to the U.N. agency.
More than 3,300 people have been killed in Kasai since the start of an insurrection in August by the Kamuina Nsapu militia, which wants the withdrawal of military forces from the area.
Kasai is the scene of a growing humanitarian disaster in a nation where violence has spiraled since President Joseph Kabila refused to step down when his mandate ended in December.
"The crisis is having a devastating impact on children," UNICEF representative Tajudeen Oyewale said in a statement.
"The lives of hundreds of thousands of children and their families in Kasai have been turned upside down by this brutal violence."
Oyewale added that the violence had disrupted access to healthcare and education in Kasai, which UNICEF described as one of the world's worst displacement crises for children.
More than 5,000 children in the region have been separated from their families, and at least 100 were killed in the first three months of the year, according to UNICEF's latest figures.
About half of militia members are estimated to be children - many younger than 15 - and more than 500 have been deployed as fighters or used as human shields, the agency said.
"The future of an entire generation of children is at risk in the Kasai region," added Oyewale.
The violence in Kasai has stoked fears of a wider conflict in the central African giant, a tinderbox of ethnic rivalry and competing claims over mineral resources. Wars at the turn of the century killed millions and sucked in neighboring countries.
Some 3.8 million Congolese are uprooted within the country, more than in any other African country, and about 7.3 million need aid urgently, according to U.N. data.
The United Nations this week named three experts to lead an international investigation into killings and other crimes in Kasai, having accused "elements" of Congo's army of digging most of dozens of mass graves discovered there in recent months.
The government has repeatedly denied its troops were responsible for the graves.
Writing By Kieran Guilbert, Editing by Emma Batha