How blessed will be the one who seizes and dashes your little ones against the rock," (Psalm 137:9).?

Can anyone explain what this means other than what it appears to be saying? Is there an alterantive meaning that I am missing?
Update: Edit: The King James version reads: Happy shall he be, that taketh and dasheth thy little ones against the stones.
Update 2: Paul B: I did but that's evolution for you :-)
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