New Defender's Study Bible Notes
137:9 Happy. This is the word usually translated “Blessed.” That is, God’s blessing would actually be upon those who would eventually overthrow Babylon and its inhabitants.
137:9 dasheth thy little ones. See notes on other imprecatory psalms (e.g., Psalms 5 and 109). The Babylonians were unspeakably cruel to God’s chosen people, desecrating the temple of God as the people carried them away as captives. In a real sense, the death of their small children, who would be safe in Christ, would be more merciful than for them to grow into adult pagans, and thereby be lost in hell forever.