"Your father and I have decided to divorce." Answer their questions honestly after that. 

His decision to leave the marriage means your days of "united" are over.  Be as nice as you can possibly be in any public situation where you and your ex will be present.  Conduct yourself with grace and manners, but you don't have to work with him or agree to let him make decisions for you anymore. Simply inform anyone involved what you will do or not do regarding plans or practical matters.  What you think of him is a matter for your private thoughts, your therapist, perhaps a few trusted friends and your lawyer.  Refrain from venting your feeling about the no good so-and-so to your children.