It's the basic social unit part. In society that actually exists, they're not a basic unit. You can obviously find couples and their dependent children, just like maybe you can find a monad in a Java program, but they're not basic units.
Kids that don’t grow up with their parents do not learn them as first words. Kids that do grow up with their parents, often still learn something else as their first words.
Learning X as your first word does not prove that X is a foundational unit of society, it simply does not follow.
I feel like you have conflated the nuclear family (a method of organising the basic social unit of a society) with "The importance of parents". The nuclear family simply isnt the only basic social unit with parents in it.
>nuclear families are so pervasively important
Parents are very important. The nuclear family does not have a monopoly on parents.