toddler vs youngster

toddler and youngster both are nouns.

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

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

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

toddler (noun)	youngster (noun)
a young child who is just beginning to walk	a young person of either sex