This article by Safe Passage for Children explains how 5 of the the 18 MN children killed by their caregivers in the last 18 months were known to local law enforcement but apparently not to child protection services (and what needs to change to fix that).
In my own experience as a volunteer CASA guardian ad Litem, my seven year old girl was prostituted for years during which the police had been to the house 49 times and only removed the child on the last call because the little girl tried to kill her younger sister in the presence of the police.