Psalms 127:3 - Matthew Poole's English Annotations on the Holy Bible

Children; which he mentions here, partly because they are the chief of all these blessings, and partly because all the forementioned toil and labour is in a great measure and most commonly undertaken for their sakes. Are an heritage of the Lord; they come not from the power of nature, and from a man's conversation with his wife, or with a multitude of wives or concubines, which Solomon had, but only from God's blessing; even as an inheritance is not the fruit of a man's own labour, but the gift of his father, or rather the gift of God, both enabling and inclining his father to give it to him. His reward; not a reward of debt merited by good men, but a reward of grace, of which we read Romans 4:4, which God gives them graciously, as Jacob acknowledgeth of his children, Genesis 33:5. And although God give children and other outward comforts to ungodly men in the way of common providence, yet he gives them only to his people as favours, and in the way of promise and covenant.