I like the idea of going to a women's shelter for some ideas. You may be surprised by some of the suggestions they have for you. They deal with scary x's all the time; they know what you can do, and do legally.
The bottom line though, is I think you just need to stand up to him, and not be afraid to make him mad. If he does get mad, then you have grounds to call the cops until he gets the message of what is acceptable or not.
Have the kids ready 15 min before pick up. As soon as he pulls up, out the door they go, and lock it if he goes to come in. He gets pissed and started banging the door down? Call the police.
If he is in the door and refuses to leave, ask him, a few times, then just walk away from him, go to another room, and call the police.
He wants to make up time he refused to take the kids because they were sick? The answer is no. He yells, screams, threatens....keep it if it's in writing, when it does it in person, call the cops.
I normally wouldn't stress to someone on here to use the police frequently, especially with children around. But with your x? You are right; he's scary. The only way to deal with scary is shut him down. Legally. With the police seeing him being an ass. With the police KNOWING you are scared of this dude.
Otherwise, I'm afraid he's going to try to control you for the next 18 years until your kids are all grown. You deserve better than that.
BS(me) 46, Two wonderful teens.
He is no longer my best friend. Repeat until it sticks.
WH says marriage is over: May 15, 2009.
EA#2 July 20, 2009. Legally sep: Aug 16, 2009. DIVORCED!!!! Signed Nov 23, final Dec 24, 2010, adultery listed.