au pair vs au pair girl

au pair and au pair girl both are nouns.

Word	Noun	Adjective	Verb	Adverb
au pair	Yes	No	No	No
au pair girl	Yes	No	No	No

As nouns, au pair girl is a hyponym of au pair; that is, au pair girl is a word with a more specific, narrower meaning than au pair:

au pair: a young foreigner who lives with a family in return for doing light housework
au pair girl: a foreign girl serving as an au pair

au pair (noun)	au pair girl (noun)
a young foreigner who lives with a family in return for doing light housework	a foreign girl serving as an au pair