Take XH at his word -- he forgot to tell you. Now, why would that happen? I'd guess that it was avoidance and/or denial. Now, why would either be involved? For some reason he felt the need to take the kids to C. My guess is that he was uncomfortable about the reason. If so, then look on the bright side -- something is sinking in and he is taking care of the kids.
Now, why are you so troubled? You are taking it PERSONALLY. Don't take it personally. His forgetfulness is not a personal slight. It is a sign of his own problems. Him not telling you did not hurt the kids. You are easily able to follow up with the C yourself. Don't pull out the legalese and pound him on the head with it, even if such a clause exists.
"Chuck, glad the kids got a chance to go to C. Thanks for taking them. You're a good dad. Let's both be more careful about giving each other a head's up about medical and C appointments -- we both need good intel on the kids, lol. Have a happy new year! Donna"
His lapse doesn't mean anything about YOU. No harm done. Indeed, be happy that he recognizes that C is important for the kids and that he recognized enough of their pain to prompt him to make an appointment.
