flower girl vs little girl

flower girl and little girl both are nouns.

Word	Noun	Adjective	Verb	Adverb
flower girl	Yes	No	No	No
little girl	Yes	No	No	No

As nouns, little girl is a hypernym of flower girl; that is, little girl is a word with a broader meaning than flower girl:

Other hypernyms of flower girl include assistant, help, helper, supporter, female child, girl.

flower girl (noun)	little girl (noun)
a young girl who carries flowers in a (wedding) procession	a youthful female person
a woman who sells flowers in the street