1)
Why would one think to withhold Musar from a youth?
1.
R. Yonah: If he is pained. Even so, do not put this to heart (even so, give to him Musar).
2.
Malbim: Even though a youth cannot receive Imrei Da'as, and one cannot bring his heart to Musar via Imrei Da'as that he will fear punishment and Hashem's grandeur. Even so, do not withhold from him Musar proper for a youth, i.e. a staff.
2)
Why does it say "Ki Sakenu va'Shevet Lo Yamus"?
1.
R. Yonah: You need not put to heart any pain [to your son] from your staff of Musar, since he will not die from it. You save him from death, like the coming verse says!
2.
Malbim: There is no danger lest he die from a staff, and it is likely that he will gain from it.
