Hiya Bluesky
I also have an N mother, although she has never been formally diagnosed. I maintain contact on a very superficial level, and pretty well always at my comfort level. I visit, I drink a cup of tea, and I come away again. I have a daughter (12) who is used to visiting her grandparents, and is in many ways close to them, but she is old enough now to realise that something is not right when she is with her gran. We talk about this and we unravel it, but I am still concerned, as others are, that I allowed too much contact when she was small.
After my ex went off the rails and into drink my mum looked after my daughter when I went to work, sometimes for as much as 3 days per week, until she started school. I hope that I am the biggest influence on her, but to be honest sometimes I wonder.
Anyway, what I wanted to say is that if you have just found out that your mother is NPD, this will result in grieving for you. You will grieve the mother you never had - as we all do - and you will go through this as with any other bereavement, and it will take a long time and be very painful for you. If at the same time you are dealing with separation from your N mum and your daughters' confusion, it may be too much.
Can you achieve emotional distance while maintaining a pretence of some form of superficial contact, or would that be too damaging? A better alternative might be to draw up a list of positive friends and relations to visit instead, and then keep yourself so busy that you and your children don't even notice that you are not seeing the N.
I am sorry you are going through such a difficult time.