Albert Barnes Commentary Psalms 127:4

"As arrows in the hand of a mighty man, So are the children of youth." — Psalms 127:4 (ASV)

As arrows are in the hand of a mighty man - They are what a parent may rely on for defense in danger, or for help in securing provision for himself and family - as the warrior or the hunter relies on his arrows.

So are children of the youth - Sons in their youth; in their prime and vigor. The comparison of sons with arrows or spears is common in Arabic poetry. See Rosenmuller, Commentary on this passage. Also Morgenland, on this passage.