I guess it would be safe to inform the police in this case especially if minors are involved and at danger...If you are too concerned about talking directly to the police I think a domestic violence hotline could also offer you good advice as they know what places need to be informed if abuse (in this case it is also abuse- it doesnt matter who the person is who abuses) happens in the context of children or teenagers in danger.
You can find the national domestic violence hotline if you google your state or country and you dont have to tell them your name if that seems too much right now.
Let us know if we can help beyond that but I think in this case it seems important the authorities know as soon as possible about this abuse before more children or teenagers become victims. They know how to handle this so that your name will not be made public if you are concerned about that. They just need to know it is happening/
Sadly it happens more often than we would think and especially abusers who would like a place of power choose professions in churches, schools etc where people generally assume he or she is GOOD. Sadly we dont check or test people enough and just assume everyone has a good heart. So many of those go decades without ever being reported because their victims fear it was their fault....after all that person is a person of trust or high prestige in their community.