There were at least two major changes that took place during this time.
First, the home stopped being the main location of economic activity for most people. This meant that children were less and less economically important to the family because they did not need to/could not work in the family business or on the family farm.
Second, the family came to be seen as an important part of creating a democratic society. This was the era of "Republican motherhood" in which women were expected to raise their children to become good workers and citizens of their democracy. Between them, these two changes meant that the family became a unit for raising and teaching children rather than an economic unit.