well.... you can either reply that you won't be coming to her house, not now, not ever... but that may be too terrifying an idea just yet
You can take the 'easier' route out in saying, "yeah, will let you know..'" and then don't..
although if she is someone who KNOWS she's not welcome in that with her gift/£20 note BS, she KNOWS you don't want her coming and she tricks you into it via your kids, she is likely to see this as a challenge and something for this dog to sink her bones into, in which case you will have to have the Hard Conversation then.
It's of course, as always up to you, this is about you. what you want, what you need, protecting yourself and your kids.
I'm not going to come and visit you mum. not now, not ever, I am bowing out of this relationship, it doesnt do me or my kids any good at all.
perhaps write her a letter you will never send? that helps somtimes too