Charles H. Spurgeon on Psalms 137:9

Happy shall he be, etc. With all possible might and speed oppose the very first risings and movings of the heart to sin; for these are the buds that produce the bitter fruit; and if sin be not nipped in the very bud, it is not imaginable how quickly it will shoot forth...Now these sins, though they may seem small in themselves, yet are exceedingly pernicious in their effects. These little foxes destroy the grapes as much or more than the greater, and therefore are to be diligently sought out, hunted, and killed by us, if we would keep our hearts fruitful. We should deal with these first streamings out of sin as the Psalmist would have the people of God deal with the brats of Babylon: Happy shall he be, that taketh and dasheth thy little ones against the stones. And without doubt most happy and successful will that man prove in his spiritual welfare, who puts on no bowels of pity even to his infant corruptions, but slays the small as well as the great; and so not only conquers his enemies by opposing their present force, but also by extinguishing their future race. The smallest children, if they live, will be grown men; and the first motions of sin, if they are let alone, will spread into great, open, and audacious presumptions. —Robert South, 1633-1716.