growing pains vs painfulness

growing pains and painfulness both are nouns.

Word	Noun	Adjective	Verb	Adverb
growing pains	Yes	No	No	No
painfulness	Yes	No	No	No

As nouns, painfulness is a hypernym of growing pains; that is, painfulness is a word with a broader meaning than growing pains:

growing pains: emotional distress arising during adolescence
painfulness: emotional distress; a fundamental feeling that people try to avoid

Other hypernyms of growing pains include pain.

growing pains (noun)	painfulness (noun)
problems that arise in enlarging an enterprise (especially in the early stages)	the quality of being painful
emotional distress arising during adolescence	emotional distress; a fundamental feeling that people try to avoid
pain in muscles or joints sometimes experienced by children and often attributed to rapid growth