The evolution of the family unit has been subtle and complex over the last few centuries. Some cultural events and occurrences have had greater impact than others. For example, the Industrial Revolution had an enormous impact upon the family unit. Prior to mechanized work in factories that came about from the rise of industry and mass production, most people worked in agriculture (farming, animal husbandry) or at tasks and jobs that could be done on the homestead (blacksmiths, silversmiths, tailors, cobblers, etc.). Working from the homestead meant children could remain at home to receive lessons or to assist in chores.
But the necessity of leaving the home to work in factories (something mostly done by the men of households) meant children were in greater need of supervision and childcare, which not could be entirely provided for by the mother (who frequently needed to take on additional duties once done by the father, such as garden/farm work or taking care of livestock). This led to the development of the public school system, and meant that children now spent the majority of their daytime hours at school, for 3/4 of the year. This had a huge impact on the structure of the American family.