Syria's civil war has killed more than 230,000 people and forced around half the population from their homes.
The United Nations says close to four million have fled Syria as refugees, mainly to neighbouring countries that struggle to cope with the influx.
The report, produced by UNICEF and Save the Children, says much more needs to be done to reverse the trend in child labour.
The report says Syria is "beset with destitution and misery", with four out of five people estimated to be living in poverty and an estimated 64.7 percent living in extreme poverty, unable to meet basic needs.
Inside Syria, it said children are contributing to family income in more than three quarters of surveyed households.
In Jordan, close to half of the refugee children are the joint or sole family breadwinners, while in some parts of Lebanon, children as young as six years old are reportedly working.
Signup to our newsletter and follow us on Facebook and Twitter!
The report finds that a "spiralling number of children are employed in harmful working conditions, risking serious damage to their health and wellbeing."
It said the most vulnerable of working children are those involved in armed conflict, sexual exploitation and illicit activities including organised begging and child trafficking.
Dr Roger Hearn, Save the Children regional director, said that "as families become increasingly desperate, children are working primarily for their survival. Whether in Syria or neighbouring countries, they are becoming main economic players."
And Dr Peter Salama, UNICEF regional director, noted that child labour "hinders children’s growth and development as they toil for long hours with little pay, often in extremely hazardous and unhealthy environments."
Moreover, there are fears of a "lost generation," as children who work are more likely to drop out of school.
The two agencies urged the international community, host governments and civil society to undertake measures to address the problem.