the Chrisitian message remains...children are better off dead.
That is not the Christian message. Matthew 19:14 gives a direct message to children: " Jesus said, "Don't stop children from coming to me! Children like these are part of the kingdom of God."
If you don't understand the Bible, then you won't understand the difference between the OT and the NT. They are completely different parts of God's plan.
Although this may seem appalling to consider it is God's right to bring judgment on whom He desires, when He desires, through whom He desires, in the manner that He desires. Man simply does not have all of the knowledge required to judge God on this issue. The amazing thing is that God was as long suffering with sinful people as He was.
It is also true that the nations God commanded Israel to destroy (i.e. Amorite, Canaanites, Midianites) had enticed Israel to sin. God desired to put an end to Idolatry. What better way to eliminate Idolatry than to completely destroy the entire Nation responsible?
Women and children were part of God's judgment in order to completely destroy the influence and Idolatry of the foreign nation.
Although Israel returned to Idolatry time and time again, the Nations judged by God were responsible for their own sin.
It matters not how or when a person dies. It matters what a person's relationship to God is. If children are "innocent" they will be judged by God as such even if they die as part of Gods judgment against the nation in which they live.
Romans 11:22
"Behold then the kindness and severity of God; to those who fell, severity, but to you, God's kindness, if you continue in His kindness; otherwise you also will be cut off."