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