foster daughter vs foster child

foster daughter and foster child both are nouns.

Word	Noun	Adjective	Verb	Adverb
foster daughter	Yes	No	No	No
foster child	Yes	No	No	No

As nouns, foster child is a hypernym of foster daughter; that is, foster child is a word with a broader meaning than foster daughter:

foster daughter: someone who is raised as a daughter although not related by birth
foster child: a child who is raised by foster parents

Other hypernyms of foster daughter include foster-child, fosterling.

foster daughter (noun)	foster child (noun)
someone who is raised as a daughter although not related by birth	a child who is raised by foster parents