What is the comparison to arrows in a strong man's hand?
Rashi: They are [effective] for fighting enemies.
Malbim: Children are Nachalas Hashem only if they are like arrows in a Gibor's hand. He does not shoot them in vain, rather, to the target and desired purpose.
What does "Bnei ha'Ne'urim" refer to?
Rashi: Talmidim that one stands up in his youth.
Radak: They are children born to a man in his youth. They are a great help. Shlomo was born in David's old age, but David had good, wise and strong sons in his youth - "u'Vnei David Kohanim Hayu" (Shmuel II, 8:18), "u'Vnei David Rishonim l'Yad ha'Melech" (Divrei ha'Yamim I, 18:17) - they were the first to help him. All were good, except for Avshalom and Adoniyahu 1 , who became haughty about their beauty 2 , and departed to evil paths. Everyone should glorify himself for good children, and considered them a good portion that Hashem gave to him.