A hyponym is a word or phrase whose semantic field is included within that of another word.
A meronym denotes a constituent part of, or a member of something
It can be said that "parents" is not just a plural form of "parent", it is lexicalized, since expresses such as "two parents", "some parents" are not normally used, and "parents" refers always to the mother and the father of someone.
The question becomes thus: Is "parent" a type of or a part of "parents"?