Best advice would be to avoid an argument (especially in front of the kids). A negative reaction could be detrimental to your case in court as well as reenforce any smack talk she said about you to the kids.
The most important thing is to be an active and good father to your kids. Be there for them whenever they need you and tell them constantly that you still love them and nothing can change that.
Speak to your attorney about any legal action you could take against her. In many states, a parent can be held in contempt for smack talking about the other parent to or in ear shot of their children.
It is the responsibility of both parents to encourage a positive relationship between their children and the other parent. In the end, it is in the best interest of your children. Good luck!