We need to look at this from a step back.
If I had received a call from my children - who were so scared they had locked themselves in a room to make the call - and neither legal parent was within earshot... AND the person with the child had prevented communication between those children and those legal parents... I would have had squad cars out front before step mom knew what was happening.
If, instead of stepmom, you were out for the evening and the kids called you to tell you that the sitter had gone nuts and they had locked themselves in a bedroom out of fear, what would you do? Would you rush home?
Mom can't... it is dad's time. I would, therefore, send those in the chain of authority that are qualified to intervene.
That would, again, be CPS. Fine, call the police instead.
DON'T wait for this to escalate before you do.