You may want to ask MNHQ to move this to the Relationships section - AIBU can be a bit fierce :) (Click Report on your post if you want to do that.) Welcome to mumsnet.
Unfortunately this kind of thing is very common, it's a control mechanism. There isn't really a lot you can do, though. Obviously don't assume that he will be reliable childcare. Try not to react to him, you will only let him see that he is getting to you. It's also a bit "dog in the manger" - he doesn't want you, but he doesn't want anyone else to have you either. He's trying to disrupt your new relationship by cancelling your alone time, basically.
How old are the children? Are they young enough you can not tell them and then be all "Oh look yay Daddy is here!" or are they expecting him? If they are expecting him, it might be worth planning some fun things that you can do if he cancels, so you can distract them quickly with "Never mind... let's go and..."
Make sure you stay positive or neutral when you talk to them about him. They will work out in their own time what he is like :( You can talk about his behaviour - and you should - reassure them that it's OK to feel sad, angry, etc if they are showing you those feelings.
Are they old enough that when he rings to cancel you can put them on the phone and get him to tell them himself in the hope it causes a guilt trip?
I really feel for you. It's the worst thing. So selfish :(