I wouldn't call the police for help if he shows up tomorrow unless he makes a scene and you feel you need their assistance for your own safety. If HE wants to call the police, let him. No cop is going to force a child to go to visitation. Just keep them with you and let him bitch and moan to the court.
I'd just keep things as steady as you can. AND I think sending the kids out for a while tomorrow night might be a good idea if he doesn't show up tonight.