schoolboy vs schoolchild

schoolboy and schoolchild both are nouns.

Word	Noun	Adjective	Verb	Adverb
schoolboy	Yes	No	No	No
schoolchild	Yes	No	No	No

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

schoolboy: a boy attending school
schoolchild: a young person attending school (up through senior high school)

Other hypernyms of schoolboy include boy, male child, adult male, man, pupil, school-age child.

schoolboy (noun)	schoolchild (noun)
a boy attending school	a young person attending school (up through senior high school)