The agency said more countries are experiencing conflict than at any time since 1989, when the UN Convention on the Rights of the Child was introduced in order to enshrine the protection of children in conflict in international law.
UNICEF said it has verified more than 170,000 grave violations of the convention since 2010, which is equivalent to more than 45 a day.
These grave violations include the killing and maiming of children; recruitment and use of children by armed forces and armed groups; sexual violence against children; attacks against schools and hospitals; abduction of children; and denial of humanitarian access for children.
More than 24,000 grave violations were verified in 2018, which is 2.5 times more than in 2010. However UNICEF points out that verification processes have improved.
More than 12,000 children were killed or maimed in conflict in 2018, and violent attacks on them have continued in 2019, according to UNICEF.
And for every violation that is verified there are many more that go undocumented, according to UNICEF Executive Director Henrietta Fore.
"Conflicts around the world are lasting longer, causing more bloodshed and claiming more young lives," said Fore.
"Attacks on children continue unabated as warring parties flout one of the most basic rules of war: the protection of children," she added.
In February, the charity Save the Children published a report on children and armed conflict.
It revealed that in 2017 some 420 million, or nearly one in five, children lived in areas affected by armed conflict or war.
This is the highest number at any time in the past 20 years, it said.