The body warns inaction now will see "unfathomably high" consequences
United Nations children's body UNICEF says almost 50 million children have been uprooted around the world.
Some 28 million of these have been driven from their homes by conflict, and millions more are migrating in the hope of finding a better life.
UNICEF says they are often traumatised by the conflicts and violence they are fleeing, and face further dangers along the way - including the risk of drowning on sea crossings, malnourishment, dehydration and trafficking.
"In countries they travel through and at their destinations, they often face xenophobia and discrimination", a new report says.
Two major summits of world leaders are due to take place in New York this month to address this issue.
The report, 'Uprooted: The growing crisis for refugee and migrant children', presents new data that "paints a sobering picture of the lives and situations of millions of children and families affected by violent conflict and other crises".
The report shows that:
"Indelible images of individual children - Aylan Kurdi’s small body washed up on a beach after drowning at sea or Omran Daqneesh’s stunned and bloody face as he sat in an ambulance after his home was destroyed - have shocked the world," said UNICEF executive director Anthony Lake.
"But each picture, each girl or boy, represents many millions of children in danger - and this demands that our compassion for the individual children we see be matched with action for all children."
UNICEF Ireland's executive director Peter Power added: "This is a shocking indictment and represents a failure of the global community to come to grips with this crisis.
"The global summits on migration in New York on the 18th/19th September will be a watershed moment.
If the global community does not act now, the consequences for the global community will be unfathomably high."
The report points to six specific actions that will protect and help displaced children: