What does it mean to say, "Everyone in this city has 2.1 kids"? Is this an idiom?
2.1 Kids: Stable Population
PS: Not necessarily reflecting my personal views.
It's a phrase in common use to mean a stereotypically average family.
There was a situation comedy on British TV a few years back called "2.4 Children" which got its title from the average size of a British family (family sizes vary from country to country). They obviously didn't have 2.4 children, unless you count the Dad acting childish 40% of the time, but the title was meant to imply that the family were average in every way.