Albert Barnes Commentary Psalms 127:3

"Lo, children are a heritage of Jehovah; [And] the fruit of the womb is [his] reward." — Psalms 127:3 (ASV)

Lo, children are an heritage of the Lord - They are an inheritance derived from the Lord. They are bestowed by him as truly as success is in building a house or in guarding a city. The idea is that everything that we value or desire is a gift from God, and is to be received as from him and acknowledged as his gift. The general idea here, as in the previous verses, is that of entire dependence on God.

And the fruit of the womb is his reward - Or rather, “a reward;” that is, they are of the nature of a reward for a life of devotion to God; they are among the blessings that God promises and are evidences of his favor. Our translation, by inserting the words “is his,” obscures the sense, as if the meaning were that they belong to God as his “reward” for what he does for us. The reverse of this is the true idea: that they are a blessing with which he rewards or favors his people. Of course, this is not universally true, but the promise is a general one, in accordance with the usual promises in the Bible regarding the result of piety.

Children are to be considered among the divine favors bestowed on us, and for their lives, their health, their virtues, and the happiness derived from them, we are, as in other things, dependent on him—as in building a house, in guarding a city, or in the rest and comfort derived from toil.