Many of the children migrants have been subjected to forced labor and prostitution. UN officials have called on the EU to create "protection corridors" for children migrants fleeing to Europe.
However, the report noted that children from sub-Saharan Africa are most at risk of exploitation, with 83 percent of those attempting to reach Europe via Libya having experienced some form of abuse.
"If you try to run, they shoot you. If you stop working, they beat," the report quoted Aimamo, a 16-year-old unaccompanied minor from Gambia, as saying. "We were just like slaves."
The UN Children's Fund (UNICEF) and the International Organization for Migration (IOM) said children from outside central and southern Africa are also subjected to exploitation, but to a lesser extent, with 56 percent reporting such incidents.
From escalating violence in the Middle East and Africa to incoherent asylum policy at home - DW looks at how the EU has found itself in the midst of a refugee crisis.
Image: picture-alliance/dpa
Fleeing war and poverty
In late 2014, with the war in Syria approaching its fourth year and Islamic State making gains in the north of the country, the exodus of Syrians intensified. At the same time, others were fleeing violence and poverty in countries such as Iraq, Afghanistan, Eritrea, Somalia, Niger and Kosovo.
Image: picture-alliance/dpa
Seeking refuge over the border
Vast numbers of Syrian refugees had been gathering in border-town camps in neighboring Turkey, Lebanon and Jordan since 2011. By 2015, with the camps full to bursting and residents often unable to find work or educate their children, more and more people decided to seek asylum further afield.
Image: picture-alliance/dpa
A long journey on foot
In 2015 an estimated 1.5 million people made their way on foot from Greece towards western Europe via the "Balkan route". The Schengen Agreement, which allows passport-free travel within much of the EU, was called into question as refugees headed towards the wealthier European nations.
Image: Getty Images/M. Cardy
Desperate sea crossings
Tens of thousands of refugees were also attempting the perilous journey across the Mediterranean on overcrowded boats. In April 2015, 800 people of various nationalities drowned when a boat traveling from Libya capsized off the Italian coast. This was to be just one of many similar tragedies - by the end of the year, nearly 4,000 refugees were reported to have died attempting the crossing.
Image: Reuters/D. Zammit Lupi
Pressure on the borders
Countries along the EU's external border struggled to cope with the sheer number of arrivals. Fences were erected in Hungary, Slovenia, Macedonia and Austria. Asylum laws were tightened and several Schengen area countries introduced temporary border controls.
Image: picture-alliance/epa/B. Mohai
Closing the open door
Critics of German Chancellor Angela Merkel's "open-door" refugee policy claimed it had made the situation worse by encouraging more people to embark on the dangerous journey to Europe. By September 2016, Germany had also introduced temporary checks on its border with Austria.
Image: Reuters/F. Bensch
Striking a deal with Turkey
In early 2016, the EU and Turkey signed an agreement under which refugees arriving in Greece could be sent back to Turkey. The deal has been criticized by human rights groups and came under new strain following a vote by the European Parliament in November to freeze talks on Turkey's potential accession to the EU.
Image: Getty Images/AFP/A. Altan
No end in sight
With anti-immigration sentiment in Europe growing, governments are still struggling to reach a consensus on how to handle the continuing refugee crisis. Attempts to introduce quotas for the distribution of refugees among EU member states have largely failed. Conflicts in the Middle East and elsewhere show no signs coming to an end, and the death toll from refugee sea crossings is on the rise.
Image: Getty Images/AFP/S. Mitrolidis
8 images1 | 8
'Abused, trafficked, beaten'
Afshan Khan, UNICEF's regional director in Europe, said that European authorities must do more to protect the livelihood of minors and young adults attempting the dangerous journey to Europe through the central Mediterranean.
"The stark reality is that it is now standard practice that children moving through the Mediterranean are abused, trafficked, beaten and discriminated against," Khan said in a statement.
"EU leaders should put in place lasting solutions that include safe and legal migration pathways, establishing protection corridors and finding alternatives to the detention of migrant children."
The long trip to Europe from the refugees' perspective
A boy who is soaking wet, volunteers entertaining children, moments of danger and of joy: #RefugeeCameras shows images taken during the journey to Europe. It is the subject of a new exhibition in Hamburg.
Image: Kevin McElvaney/ProjectRefugeeCameras
Leaving home - for good?
Zakaria received his camera on December 8 in Izmir, Turkey - one of the key hubs for refugees. The Syrian fled from the "Islamic State" terror militia and the government, according to McElvaney's #RefugeeCamera project. Out of safety concerns, Zakaria doesn't name his hometown. In his flight diary, Zakaria writes that only God knows if he will ever be able to return to Syria.
Image: Kevin McElvaney/ProjectRefugeeCameras
Rough dinghy ride
Zakaria documented his sea journey from Turkey to Chios. He was sitting in the back of his dinghy. At the Hamburg exhibition, which opens this weekend, the refugees' images will be complemented by a selection of shots taken by professionals, who helped to shape the representation of escape routes in the media. They all donated their works in order to support the project.
Image: Kevin McElvaney/ProjectRefugeeCameras
Perilous arrival
Hamza and Abdulmonem, both from Syria, photographed the perilous landing of their dinghy on a Greek island. There were no volunteers to offer them support. That is exactly what McElvaney had in mind when he launched #RefugeeCameras. So far, he says, the media have offered a "visual blank" in this respect.
Image: Kevin McElvaney/ProjectRefugeeCameras
Surviving the sea
After the landing, a young boy in wet clothes and life jacket stands on the pebbled beach. The image brings to mind Aylan Kurdi, the small Syrian boy whose lifeless body was washed ashore on a Turkish beach in September. The child in this picture made it to Europe alive. What became of him is not known.
Image: Kevin McElvaney/ProjectRefugeeCameras
Seven cameras returned
Hamza and Abdulmonem also took this slightly blurred snapshot of the refugee group taking a break. McElvaney handed out 15 disposable cameras in total. Seven of them were returned, one was lost, two were confiscated, two remain in Izmir, where their holders are still stranded. The remaining three cameras are unaccounted for - just like their owners.
Image: Kevin McElvaney/ProjectRefugeeCameras
Family in focus
Dyab, a math teacher from Syria, tried to capture some of the better moments of his journey to Germany. Pictured here are his wife and his young son, Kerim, who shows us the packet of biscuits he was given in a Macedonian refugee camp. The images reveal Dyab's deep affection for his son, McElvaney says: "He wants to take care of him, even on this arduous trip which he was forced to take."
Image: Kevin McElvaney/ProjectRefugeeCameras
From Iran to Hanau
The story of Saeed, from Iran, is a different one. The young man had to leave the country after converting to Christianity. He could have been arrested or even killed. In order to be accepted as a refugee, he pretended to be Afghan. After his arrival in Germany, he explained his situation to the authorities' satisfaction. He now lives - as an Iranian - in Hanau, Hesse.
Image: Kevin McElvaney/ProjectRefugeeCameras
Beyond selfies
Saeed took this picture of a Syrian father and his child on a bus from Athens to Idomeni.
Image: Kevin McElvaney/ProjectRefugeeCameras
More than status
In another snapshot taken by Saeed, a volunteer working in a refugee camp somewhere between Croatia and Slovenia entertains a group of children, who try to imitate his tricks.