child vs youngster

child and youngster both are nouns.

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

As nouns, child and youngster are synonyms defined as:

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

child (noun)	youngster (noun)
a young person of either sex	a young person of either sex
a human offspring (son or daughter) of any age
an immature childish person
a member of a clan or tribe