I have a friend whose mother was NC with her own father. Friend got to her teens and asked her mother about her grandfather, and persuaded her mum to get back in touch, which she did (against her better judgement). Didn't take long for friend to realise her mistake, but yes, once that can of worms was opened, it was a very difficult task to contain it all again! She regretted asking her mum to do it.
And that was nothing like as bad a situation as your mother put you through.
You say your children know all about the abuse - all about what your mother did to you. Maybe they want to ask her why, how she could do that? Maybe, if they're lovely children, they absolutely cannot understand how anyone COULD do that to their child, so want to take her to task over it.
But. Yes, again, once that can of worms is opened...
You've been very open with them so far. Tell them that any rapport between them and your "mother" would hurt you, feel like a betrayal, cause you pain. And then tell them that you would understand if they still felt the need to be curious and contact her. You will still love them (of course you will!) despite your pain. Tell them as well that you do NOT want to hear anything about her, AT ALL.
Totally understand your feelings over this though, totally. Just trust your children to be the people you have brought them up to be
x