kiddy vs youngster

kiddy and youngster both are nouns.

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

As nouns, youngster is a hypernym of kiddy; that is, youngster is a word with a broader meaning than kiddy:

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

kiddy (noun)	youngster (noun)
informal term for a young child	a young person of either sex