foster child vs foster-child

foster child and foster-child both are nouns.

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

As nouns, foster child and foster-child are synonyms defined as:

Other synonyms of foster child include fosterling.

foster child (noun)	foster-child (noun)
a child who is raised by foster parents	a child who is raised by foster parents