Oh love, I've been in your shoes, it's soul destroying isn't it? And heartbreaking to see the kids so upset each time he let's them down.
Unfortunately even with a court order he cannot be forced to see the children. (And no judge would force dc to see a parent who didn't want to see them). Don't waste your money applying to court.
So it's all about taking back some of the control here - the kids need stability, and routine, and you all need to not be constantly waiting for his call as to when he feels he is capable of seeing the kids.
So. Text or email only from now on with regards to making arrangements for the dc. Do not slag him off to the dc - tempting, but don't. Begin with a message explaining how unsettling it is for the dc not to know when they will see him. Tell him that in order to maintain a routine FOR THE DC you are happy to facilitate contact at x times/days. Give him an opportunity to offer alternative set times/days for this new arrangement. Then, and this is the important part, stipulate that in the interests of the children's wellbeing, if he is not available for one contact, then their next contact will be the next designated time (unless dire emergency/pre-arranged). It will mean some discussion with the dc, and inevitable disappointment for them when he let's them down. (Yes, I've spent hours with my dc mopping up tears when daddy didn't turn up. It's shit. But ensure you are consistent with your message that daddy wasn't able to make it, you don't know why, you'll hopefully see him next time etc).
This way, the children know when contact should be. None of you are constantly waiting for a call.
You are NOT denying him contact with the dc. You ARE creating a workable routine which is in the best interests of the dc. He CANNOT claim to ANYONE that you are blocking contact (though he will try) and you are absolutely doing the right thing for the dc (and yourself). It is going to be tough. The kids will work it out eventually op - that you're helping them to maintain a positive relationship with their dad, but it is him that is letting them down.
I've been separated/divorced for 12 years. It took me 4 years to get to this point, and it was utterly soul destroying. As soon as I took back the control, he stopped messing us all around. Sadly the kids (now late teens) barely see him but have a realistic opinion about his parenting.