An assembly line is a manufacturing process (most of the time called a progressive assembly) in which parts. Usually interchangeable parts are added as the semi-finish assembly moves from work station to work station where the parts are added in sequence until the final assembly is produce. A finished product can be assembled faster and with less labor then by having workers carry parts to a stationary piece for assembly.
Child labor reached new extremes during the industrial revolution. Children often worked long hours in dangerous factory conditions for very little money. Children were useful as laborers because their size allowed them to move in small spaces in factories or mines where adults couldn't fit.