Can you just leave him out of the arty plans because he continually ruins his childrens birthdays?
I understand that it is about the kids, and their big day, but if dad turns it into a $h!tshow, wouldn't it be better if he wasn't there? My brother and his ex couldn't get along for a long time - to the detriment of the kids. They both had 2 birthdays, and 2 christmases every year. Not ideal - but saved a whole lot of drama and mess.