Has HE told you he is not going? You are working on third hand information. (I agree with the others, Invite the Ex wife and the girls if he does not want to go). Just make sure to get a final answer from him. Tell him that you are thinking of inviting someone else if he is going to bail on you. Since he has never told you a definite no, Make sure the girls are not mistaken about not going. Or the Ex wife is not trying to cause problems between you and him. He may have planned on suprising them at the last minuite. (unlikely, but you need to make sure before you take action)