child vs nestling

child and nestling both are nouns.

Word	Noun	Adjective	Verb	Adverb
child	Yes	No	No	No
nestling	Yes	No	No	No

As nouns, child and nestling are synonyms defined as:

Other synonyms of child include fry, kid, minor, nipper, shaver, small fry, tiddler, tike, tyke, youngster.

child (noun)	nestling (noun)
a young person of either sex	young bird not yet fledged
a human offspring (son or daughter) of any age	a young person of either sex
an immature childish person
a member of a clan or tribe