Dear God, whatever you do, do not send the school board or Superintendent letters regarding your specific children. It is a waste of paper. The school can not imped an investigation. That is not within their authority. Same with the police or other law enforcement agency. DCFS may speak to the children at school or anywhere else and need not inform you first. Think about it. They often are investigating whether or not a parent is abusing a child. Why on earth would they question the child in front of the abuser? How honest do you think the kid is going to be with their abuser sitting there?
You can instruct your kid not to cooperate but that is ultimately not going to help you any. DCFS is not going to just close their case and walk away because your kid says you told them not to cooperate.