The report, entitled Hidden in Plain Sight, said that children were often abused in their own communities, homes and schools - much of it going unreported.
Drawing on data from 190 countries, it noted that a fifth of murder victims globally in 2012 were under the age of 20.
The violence "cuts across boundaries of age, geography, religion, ethnicity and income brackets," UNICEF Executive Director Anthony Lake said in a statement. "It occurs in places where children should be safe."
The United Nations body, which in June urged governments to do more to protect the rights of children, also noted that some 1 in every 10 girls worldwide had been raped or forced into some other form of sexual abuse. UNICEF experts blamed entrenched social attitudes about gender and the rearing of children as being partly to blame for the figures.
"What we didn't know until now was the extent of the problem," said UNICEF Deputy Executive Director Geeta Rao Gupta.
"Too many victims, perpetrators and bystanders see it as normal, and when violence goes unnoticed and unreported we fuel the belief among children that it is normal."
Enduring effects of ill-treatment
While shocking, Gupta said, the figures are thought to underestimate the levels of abuse, with many children unwilling to report violence against them, as well as its long-term effects.
"Violence against children, direct or indirect, can cause trauma, low self-esteem, bad health and poor school performance, and in some cases, it can lead to depression, alcohol and drug abuse, self-harm and even suicide," Gupta said.
The report found that homicide was the leading cause of death among males between 10 and 19 in many Latin American countries, including Brazil, El Salvador, Guatemala, Panama and Venezuela.
Nigeria, where an Islamist terrorist group kidnapped more than 200 schoolgirls in April, had the largest number of young murder victims - almost 13,000 in 2012.
rc/lw (AP, Reuters)