Since it was clipped and I never got to say my piece...
God ordered the death of those nations and children for one simple reason, survival. It was a military reason. God was teaching Israel's leaders and even commanded them, when defending from attack or when invading another nation not to allow anyone to live or when they grow up their descendents will seek revenge.
God taught Israel, as a nation, in order to survive that when no enimies, current or future are left behind there would not be residual attacks.
We need to remember that Israel was a real nation really living amongst hostile nations and they had an army and were attacked and attacked others. God, who gave them intsructions on all things, also gave intruction concearning thier war tactics.