Psalms 127:3 - Joseph Benson’s Commentary on the Old and New Testaments

Lo, children are a heritage of the Lord They come not from the power of mere nature, but from God's blessing, even as an inheritance is not the fruit of a man's own labour, but the gift of God. He can, in a moment, blast the most fruitful stock, or he can make the barren woman keep house, and become a joyful mother of children. The psalmist mentions children here, because all the forementioned toil and labour are, in a great measure, and most commonly, undertaken for their sakes; and because they are the chief of all those blessings to which he refers. And the fruit of the womb is his reward Not a reward of debt, merited by good men, but a reward of grace, as the apostle expresses himself, Romans 4:4, which God gives them graciously, as Jacob acknowledges of his children, Genesis 35:5. God indeed frequently gives children, and other outward comforts, to ungodly men, but this is in the way of his common providence; whereas he gives them to his people as peculiar favours, and in the way of promise and covenant.