ALSO my advice is to NOT disparage him in front of the kids. Kids are smart, they know what's up, they don't need color commentary from parents.

My wife's mom to this day cannot even hear her real father's name.... it causes a major strain between my wife and her mom, because her mom's portrayal of her bio dad couldn't be farther than the truth. The bio dad is a BETTER grandparent than the bio mother of my wife.... so even if there is a grain of truth to what you're saying, your child's relationship with your soon to be ex will be different than yours. It's actually quite inappropriate for you to discuss the issues you're having with your ex in front of the kids ever.... it's really none of their business, and in the end you really don't want to do anything that will cause your children to NOT want a relationship with you. So make sure you do the best to support what your children want, not what you want!