schoolchild vs spring chicken

schoolchild and spring chicken both are nouns.

Word	Noun	Adjective	Verb	Adverb
schoolchild	Yes	No	No	No
spring chicken	Yes	No	No	No

As nouns, spring chicken is a hypernym of schoolchild; that is, spring chicken is a word with a broader meaning than schoolchild:

schoolchild: a young person attending school (up through senior high school)
spring chicken: a young person (especially a young man or boy)

Other hypernyms of schoolchild include young person, younker, youth.

schoolchild (noun)	spring chicken (noun)
a young person attending school (up through senior high school)	a young chicken having tender meat
	a young person (especially a young man or boy)