I so appreciate and understand your frustration with everyone who had something to do with allowing these precious children to be victimized and murdered by their own Father. I do not blame the State, nor LE, nor the visitation supervisor, nor a failure in our system. I blame Josh Powell for this horrendous tragedy.
Laws in this country are based upon the assumption that everyone is innocent until proven guilty. There was nothing else that DCFS could do (IMO) in that JP was never charged with Susan's disappearance, murder (?), or even child endangerment when he admitted he took the toddlers camping in a snowstorm (IIRC). CPS could not take away his rights to see his children unless there was evidence he was a significant danger to his children. By not allowing him supervised visits . . . that essentially relates to severing a parental/child relationship without the appropriate legal "termination of parental rights" procedures which allows the parents opportunities to ameloriate the issues that brought the children into (foster/kinship) care.
Furthermore, the children have the right to see their parents as well. For those of you who have followed the Sky Metalwala case, (also in Washington State) we were all rooting for little MM to go home to her Daddy, Solomon (and still are thankful). MM (age 4) was placed into fostercare when her Mom reported little brother Sky (2 1/2) went missing from her car when she left him alone, claiming she ran out of gas. No one had seen Sky for many weeks (if not months) previously. He still has not been found. Solomon's rights to visits with his children were revoked until he completed expensive classes for domestic battery, based solely upon mother, Julia's allegations.
Both cases compare and contrast how CPS and family courts attempt to balance the needs and safety of the child vs. parental rights.