Lisa- It's okay that he filed. In a sense he is going to force the issue with custody and frankly that is in your favor. Keep doing what you are doing. You might ask the resource officer at the school if they are aware of any counseling programs - free or low cost for your kids- I was surprised at the resources I found. Your H is most definitely still trying to control. You are holding up like a champ! Don't take the bait, don't get sucked in. Obviously, he DOES NOT have the kids best interest in mind or he wouldn't be forcing OW on them.
You can do this. Remember if things actually change that much you can always stop the D later if you should reconcile.