"Ruthless smugglers and traffickers are exploiting their vulnerability for personal gain, helping children to cross borders, only to sell them into slavery and forced prostitution", Mr Forsyth said.
One 17-year-old girl from Nigeria told Unicef that she was trapped in Libya for three months and sexually assaulted by her smuggler-turned-trafficker as she attempted to travel alone to Italy.
Around 300,000 unaccompanied and separated children were documented by UNICEF in 2015 and 2016 compared to only 66,000 in 2010 and 2011.
Of the 300,000, some 100,000 were caught trying to cross the border from Mexico to the US, UNICEF said. Almost all minors who made it across the Mediterranean from Libya to Italy previous year and so far in 2017 had arrived alone."Deprived, unprotected, and often alone, children on the move can become easy prey for traffickers and others who abuse and exploit them", UNICEF said.
It said over half the total, 170,000, requested asylum in Europe, and 92% of the children who have arrived in Italy were unaccompanied or had become separated from their parents.
"One child moving alone is one too many, and yet today, there are a staggering number of children doing just that - we as adults are failing to protect them", said UNICEF Deputy Executive Director Justin Forsyth yesterday in a news release.
"We must build upon the work done to bring children to safety from Calais a year ago and improve our rules on family reunion".
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Most children from Central America migrate to escape gang violence and poverty, or to join relatives living overseas.
While some of the unaccompanied children are orphans, others are seeking to join relatives who already reached prosperous countries.
Speaking from the largest refugee camp in the world in Uganda, World Vision chief advocate, Tim Costello, said the organisation has registered thousands of children travelling alone in the last 10 months.
Unaccompanied migrant and refugee children should not be placed in adult detention facilities and ideally should be in foster care, the report said.
On a global scale, child victims of trafficking make up about 28% of unaccompanied minors who arrive in Europe, the report said.
The law includes a strict prohibition on turning unaccompanied minors away at the border and guarantees them access to healthcare.
Among those hailing its passage at the time were UNICEF and the humanitarian group Save the Children.