Well said.

This may sound odd, but when it comes to this sort of situation, I find it helpful to take some time (meditate maybe?), and try to mentally recategorize the person in question. Move them from who they were, to more of who they are now. Maybe in your case, you can start to think of her as something like an old annoying neighbor that you don't really want to run into. Try to recategorize her as someone to whom you're indifferent and detached.

I saw a great video a few days ago about boundaries: the presenter talked about how narcissists bait and need the reaction from you. Without your reaction, they are nothing. As he said, "You don't fight for what you don't value. If the narcissist can't get a reaction out of you, you've just told them they have no value." Non-reaction is power over a narcissist.

It might feel weird, but it can help to practice lines you might use- "Excuse us, we're in a hurry." , "I've got to go." , "I'm not going to discuss that here.". Simple phrases that put down your boundaries, but don't go so far as to be placating in any way, or feel like you're justifying yourself.

Ultimately, it boils down to taking care of the needs of you and your kids- act on that in the moment, and give zero mind to anything she's doing or wants.