Nearly 50 million children around the world have been driven from their homes by conflict or migrated across borders, according to a major new report.
More than half of these (28 million) were forcibly displaced by conflict and violence within and across borders, while a further 20 million children fled their homes for other reasons including to escape gang violence and extreme poverty, research by Unicef estimates.
The charity claimed there is a "disproportionate and growing" number of uprooted children at risk of "falling through the cracks" because they have no documentation and an uncertain legal status.
As well as the dangers encountered during their journey, children often face xenophobia and discrimination and are more likely to miss out on services including health and education when they reach their destination country, the report, Uprooted: The Growing Crisis For Refugee And Migrant Children, warned.
Lily Caprani, Unicef UK's deputy executive director said: "Today, nearly one in every 200 children in the world is a refugee.
"In the last few years we have seen huge numbers of children being forced to flee their homes, and take dangerous, desperate journeys, often on their own.
"Children on the move are at risk of the worst forms of abuse and harm and can easily fall victim to traffickers and other criminals.
"Many of these children wouldn't resort to such extreme measures if the UK Government made them aware that they may have a legal right to come to the UK safely and if they provided the resources to make that process happen before these terrible journeys begin."
The number of child refugees under the United Nations High Commissioner for Refugees' (UNHCR) mandate has more than doubled in the 10 years up to 2015 - a statistic the charity branded "shocking" and "unacceptable".
More than twice as many children applied for asylum within the European Union (EU) and free movement zone in 2015 than the previous year.
Last year there were 88,265 asylum claims by unaccompanied children in the EU, of which only 3,045 were in the UK (3.4%), according to the statistics body Eurostat.
Unicef is urging the UK to show "vital leadership" at two refugee and migrant summits in New York later in September.
It is calling for the UK Government to do more to ensure that refugee children stranded in Europe can reach safety with their families in the UK, and offer more resettlement places to refugee children facing dangers.
The number of child asylum seekers being cared for by councils in England has risen by more than half since last year, figures released to the BBC show.
By the end of March there were at least 4,156 unaccompanied children seeking asylum and in the care of councils, the broadcaster said - a rise from 2,569 a year earlier.
The three councils with the largest number of child asylum seekers were Kent, Croydon and Surrey, freedom of information requests showed.
The majority of the lone children were from Afghanistan, followed by Eritrea and Albania.
On Sunday the Government announced it had secured enough local authority places across the UK to resettle 20,000 Syrian refugees over the next four years.