Nearly 50 million children have been driven out of their own countries by conflict or migrated across borders, according to a new report.
More than half of these (28 million) were displaced by conflict and violence within their home country and across borders, with an additional 20 million children fleeing to escape things like gang violence and poverty, Unicef research estimates.
The children's charity said there is a "disproportionate and growing" number of displaced children at risk of "falling through the cracks" because they have no legal status or documentation.
As well as the risks they face at home and while leaving, children face further discrimination and xenophobia when they reach their destination country, the report warned.
Unicef UK's deputy executive director, Lily Caprani, 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.
"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."
Unicef is urging the UK to show "vital leadership" at two migrant and refugee summits in September, asking the government to ensure refugee children can reach safety in the UK, and to offer more resettlement places.
The call comes a few days after the Home Office announced that it has successfully found enough local authority places for 20,000 Syrian refugees over the next four years.