I think you are being more than fair. If the plan was for them to see their grandmother and they want to do that, then that's what they should do. It's all about the kids, not him and not you. If he has a habit of bailing on plans, then he has absolutely no reason to complain. I'd just tell him that the kids want to go to the zoo and that's what they will be doing. If he wants to pick them up afterward, fine. Let it go at that. If he gets upset, he can just get glad in the same pants he got mad in.