Children working in factories was only a transitory stage between early feudalism and capitalism. Prior to working in factories, before capitalism, many of children (and their parents) used to die and starve, as evidenced by the high infant mortality statistics before capitalism. Observe that is was not until families left the “country” and went into the “cities” that they were able to produce enough food to eat. The clearest evidence of this is population and infant mortality statistics: population did not go up, and infant mortality did not go down, until the Industrial Revolution. If life was so great before capitalism in the “country”, why was infant mortality so high and population numbers considerably lower before capitalism? Answer: because life was not so great until Capitalism.
Throughout history the parents of most families could not produce enough to support their families without having their children work also (such was the case of my father in India). It was the accumulation capital by the industrialists that made the labor of parents more productive, that children had to stop working in fields or factories. In poor non-capitalist countries they are still working in fields and factories.
Contrary to leftist rhetoric passing child labor laws in these countries will not solve the problem, but will only lead to mass starvation — which is why the “poor” themselves resist such laws (it is only to the benefit of the leftist “rich” “humanitarians” who cry out for them).