child welfare service vs child welfare agency

child welfare service and child welfare agency both are nouns.

Word	Noun	Adjective	Verb	Adverb
child welfare service	Yes	No	No	No
child welfare agency	Yes	No	No	No

As nouns, child welfare service and child welfare agency are synonyms defined as:

child welfare service and child welfare agency: an administrative unit responsible for social work concerned with the welfare and vocational training of children

child welfare service (noun)	child welfare agency (noun)
an administrative unit responsible for social work concerned with the welfare and vocational training of children	an administrative unit responsible for social work concerned with the welfare and vocational training of children