Professor Jane Humphries wrote a book about child labour in 19th century Britain, one which I unfortunately no longer have in my possession, called Childhood and Child Labour in the British Industrial Revolution. If I recall it correctly, there were up to a million children (under the age of eleven) in work, full time, during this century at any one time. These were children working in factories and industrial labour. There are several reasons why, but they basically come down to survival; the additional, if lesser, income of your children could stand between a family and starvation.
Child labour was progressively restricted in legislation during the 19th century, which was accompanied by the introduction of mandatory schooling and progressive rise in leaving age; it finally ended in the mid-20th century when both combined to restrict working hours and set a school leaving age at the point of 'adulthood', or at least mid/late teens.
