The figure includes children uprooted from Venezuela, those who have returned and others in host and transit communities across Latin America and the Caribbean, said a UNICEF statement.
The latest estimate highlighted expectations among aid agencies that the political crisis in Venezuela will worsen in the coming months, exacerbating the dire humanitarian situation of children and families.
UNICEF called on governments in the region to uphold the rights of children and ensure they have access to essential services.
An internal U.N. report seen by AFP last week said seven million people -- about 24 percent of Venezuela's population -- are in need of humanitarian aid, lacking access to food and medical care.
President Nicolas Maduro has blamed U.S. sanctions for Venezuela's economic problems but opposition leader Juan Guaido, who has declared himself interim president, says government corruption and mismanagement are at fault.
Guaido is recognized by the United States and around 50 other governments.
The U.N. Security Council is expected to meet next week at the request of the United States to discuss Venezuela's humanitarian crisis.