"You know, Walt loved the idea of progress and he loved the American family. He himself was probably as American as anyone could possibly be. He thought it would be fun to watch the American family go through the twentieth century experiencing all new wonders as they came. And he put them together in a show called Carousel of Progress, which we are about to see."
This is just perfectly vague.
It does not say FOUR familes, nor does it say ONE family. It says THE AMERICAN FAMILY.
Notice the first sentence "Walt loved the idea of progress and he loved the American family."
This does not mean Walt loved ONE American family and the rest of them can go off. It means he loved the IDEA of the American family and what it represents.
The family in the CoP represent THE AMERICAN FAMILY. Some aspects from era to era are the same, so you can see how innovations effect similar people, in simplar situations, in different times. However they are also small changes, because it is not one family, but a representation of ALL American Families.
-dave