It sound to me like his kids are playing the guilt card. They have learned they can make their dad jump whenerver they want and they are taking full advantage of him. You do not have to accept ths behaviour. I would not tell him to make a choice between you or his children because I assure you, you will lose. I would tell him that you expect him to keep his dates with you or give you a minimum of a day's notice if he has to cancel. He should understand that you also have a life besides the time you spend with him. Even if you just stay home and read a book, you deserve better treatment.