street child vs waif

street child and waif both are nouns.

Word	Noun	Adjective	Verb	Adverb
street child	Yes	No	No	No
waif	Yes	No	No	No

As nouns, street child and waif are synonyms defined as:

street child and waif: a homeless child especially one forsaken or orphaned

street child (noun)	waif (noun)
a homeless child especially one forsaken or orphaned	a homeless child especially one forsaken or orphaned