Jesus himself said those who harm children should be drowned in the sea.
Are you arguing with Jesus?
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Jesus didn’t say that. He is quoted as saying ‘it would be better’ for them to be drowned.
He could have been speaking of divine retribution for their acts, not that they SHOULD be drowned.
The Old Covenant was in play until Jesus’ death and resurrection. Jesus is the one who said the Old Covenant of the Law was replaced by the New Covenant of Grace (Hebrews 8 & 10). Seems to me, you are the one who is arguing with Jesus.