"Where's your evidence for this claim? And even if true, how does that justify killing toddlers?"
This is a good starting point: http://www.theology.edu/canaan.htm
If you don't want to read the biblical side and just the purely historical side skip to "Canaanite Morality (an oxymoron)"
Basically, disease, child sacrifice, pedophilia (which caused the children to spread deadly disease), etc...etc...
I'll leave you to find the rest.
If you knew a child had an unstoppable disease that would kill millions, would you kill them or would you let them spread the disease?