I've not been where you are; my only advice is 2nd hand from watching too many friends have to navigate those treacherous waters.  Here goes:  no matter how much of your tongue you have to bite off to keep it from happening, never ever bad-mouth their mother to your children and don't use the children as weapons in the fight.  No matter how hard it is to do now, the rewards you will reap later will far outweigh the immediate satisfaction of saying what you're thinking.

Good luck, and God bless.