1)
Why does it say "Lechu"?
1.
Radak: Sometimes (including here) this refers to acting zealously, but not necessarily walking, e.g. "Lechah Na Anasechah v'Simchah" (Koheles 2:1).
2)
Which "Banim" does he address?
1.
Radak: You who hear and guard my Musar and teachings. Also Bnei ha'Nevi'im (Melachim I, 2:3) are Talmidim of Nevi'im.
2.
Malbim: They are even minors; anyone can attain Yir'as Hashem. This is unlike Nochrim, for whom Kedushah and Yir'ah are only for their great ones and priests.
3)
Why will he teach to them Yir'ah?
1.
Radak: He mentioned that Hashem's angel surrounds Yir'ei Hashem (verse 8), and they do not lack anything (verse 11).
