Short answers, pulled from the story.
Child labour is any work children engage in that interferes with their ability to attend regular school, or that is mentally, physically, socially or morally harmful. It is prohibited by legislation worldwide, though these laws exclude some work such as that of child artists, family duties, supervised training, and certain work by Amish children and by Indigenous children in the Americas.
UNICEF and the ILO estimated that 168 million children aged 5 to 17 were involved in child labour in 2013. In the world's poorest countries, around one in five children are engaged in it, and in sub-Saharan Africa more than one in four children are.
The ILO suggests that poverty is the single greatest cause of child labour. For impoverished households, income from a child's work is often crucial for survival and can amount to between 25 and 40 percent of total household income. A lack of affordable, quality schools is another major factor.
Agriculture is the largest employer of child labour worldwide, accounting for about 60 percent of it through farming, dairy, fisheries and forestry. Most child labour occurs in rural areas, around 70 percent, and children are predominantly employed by their parents rather than in factories.
During the Industrial Revolution that began in Britain in the late 18th century, children as young as four worked in factories and mines under often fatal conditions, and boys that age worked as chimney sweeps. In 1788, two-thirds of the workers in 143 water-powered cotton mills in England and Scotland were described as children, earning as little as 10 to 20 percent of an adult man's wage.
The United States is the only nation that has not ratified the Convention on the Rights of the Child. Adopted by the United Nations in 1990, the convention defines a child as anyone below the age of 18 and is now joined by 196 countries.