This marks a dramatic escalation of a trend that has forced many young refugees into slavery and abuse, UNICEF said on Wednesday.
The United Nations Children's Fund counted 300,000 unaccompanied and separated children worldwide in 2015 and 2016, up from 66,000 in 2010 and 2011.
"A Child is a Child: Protecting children on the move from violence, abuse and exploitation" presents a global snapshot of refugee and migrant children, the motivations behind their journeys and the risks they face along the way. Border closures, aggressive pushback measures, overcrowded shelters, makeshift camps and heavy-handed authorities have only served to exacerbate the risk of child exploitation, encouraging unaccompanied minors to take highly risky routes in a desperate bid to reach their destinations.
"One child moving alone is too many, and yet today, there are a staggering number of children doing just that - we as adults are failing to protect them", Forsyth said.
When they can't find opportunities to move legally, children sometimes have to resort to unsafe routes and engage smugglers to help them cross borders.
"It is unconscionable that we are not adequately defending children from these predators", he continued.
Those who survived the journeys recounted harrowing stories of abuse along the way, including a 17-year-old girl from Nigeria who told officials that she was raped in Libya by a man who had promised her passage to Europe.
"Now the people who paid for my trip are saying to my mother, it's time for money", she said. "It was a lie". Despite promising to keep her safe, she ended up stuck in Libya for more than three months.More news: Trump willing to use engagement on N.Korea crisis -S.Korea envoy
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The report says ninety-two percent of minors who arrived in Italy in 2016 were without an adult.
Children account for approximately 28 per cent of trafficking victims globally.
Children make up just under 30 per cent of human trafficking victims around the world, but in Sub-Saharan Africa and Central America, that number is above 60 per cent.
UNICEF said the central Mediterranean route between north Africa and Italy is one of the world's deadliest, with 4,579 deaths a year ago, including some 700 children, many from Eritrea, Gambia, Nigeria, Egypt and Guinea.
"These children need a real commitment from governments around the world to ensure their safety throughout their journeys", said Forsyth.
With a G7 summit taking place in Italy later this month, Unicef is calling on all governments to give refugee and migrant children better protection by adopting a six-point agenda.
UNICEF promotes the rights and wellbeing of every child, in everything we do.