How do you explain what a narcissistic parent is to your young child? How do you cope with constantly having to defend yourself as a parent to your child due to the narcissistic parent?

Edit: I've already left. I'm just forced to coparent with them.

How do you tell them the truth without pointing out the other parent is a liar. I've never told my child he's a narcissist. I've had a hard time defending myself. I try really hard not to say anything negative about him but I'm essentially pointing out he's a liar constantly.