sonny boy vs male child

sonny boy and male child both are nouns.

Word	Noun	Adjective	Verb	Adverb
sonny boy	Yes	No	No	No
male child	Yes	No	No	No

As nouns, male child is a hypernym of sonny boy; that is, male child is a word with a broader meaning than sonny boy:

Other hypernyms of sonny boy include boy.

sonny boy (noun)	male child (noun)
a male child (a familiar term of address to a boy)	a youthful male person