"Sons/Children of ..." (בְּֽנֵי־) is a common way to indicate a nation. For instance, the Israelites are literally the sons of Israel, the Ammonites the sons of Ammon, etc. The sons of man is a term for all of mankind.
In Prov 15:11, they are not worse than Sheol and Abaddon. The meaning of "lie open" (נגד) is that YHWH has surveillance and knowledge of Sheol and Abaddon. If his power extends to the realm of death, then surely he is also able to see into the hearts of men, is the reasoning.
As Waltke (The Book of Proverbs, Chapters 1–15) notes (pp. 622–623):
As in 11:31, the proverb uses the a fortiori argument. God's surveillance extends to the realm of the dead [...]. How much more transparent to him are human hearts [...]. His omniscience of the human heart and the connection of that knowledge with the grave give the argument a threatening tone. Sheol [...] is a poetic word for the grave. And Abaddon (wa'abaddôn; [...]) rhetorically intensifies the grave as a place of destruction; the root 'bd means "to perish." (Abaddon speaks as a personification of death in Job 28:22, and in Rev. 9:11 is the name of the Angel of the Abyss.)