The wars God's people fought at his command against his enemies were to remove a people who practiced child sacrifice and other detestable practices. These people were destroyed for their evil conduct.
There is little evidence the Canaanites actually practiced child sacrifice. Archeology shows the Canaanite culture was almost indistinguishable from that of the Israelites. Mark Smith in "The Early History of God: Yahweh and Other Deities of Ancient Israel" states "Despite the long regnant model that the Canaanites and Israelites were people of fundamentally different culture, archaeological data now casts doubt on this view. The material culture of the region exhibits numerous common points between Israelites and Canaanites in the Iron I period (ca. 12001000 BC). The record would suggest that the Israelite culture largely overlapped with and derived from Canaanite culture. "
If you want to talk about the killing of children and other genocide though, look no further than:
Deuteronomy (2:34) "So we captured all his cities at that time and utterly destroyed the men, women and children of every city. We left no survivor."
Deuteronomy (7:2) "and when the LORD your God has delivered them over to you and you have defeated them, then you must destroy them totally. Make no treaty with them, and show them no mercy."
Joshua (6:21) "And they utterly destroyed all that was in the city, both man and woman, young and old, and ox, and sheep, and donkey, with the edge of the sword."