OP, I've been nc with my mother for nearly 13 years.
My children are now 18 and 26. They were 6 and 13 when I did it.
I did it for a combination of reasons. Like you, I was already LC with her and the children had spent very little time with her without supervision. Nearly 13 years ago, something happened that caused the police and SS to be very briefly involved and the outcome of that was going nc. Due to the nature of it, I wasn't actually allowed to tell the children why I had gone nc with her. The children are aware it was serious - my son took a call from the police and they both spoke with the SW.
They still don't know. One day, I will be able.to tell them but l, as yet, I can't.
I gave them part of the truth in an age appropriate way. I have talked about how things were when I was growing up but vaguely and without detail. It was something they brought up periodically and I gave them more detail as they were older. It's so different to their experience that they can't really comprehend it and the youngest has thrown back at me that she will go nc with me for very basic parenting things like wanting her to keep her room tidy! But that's just normal.
They will have questions and that's OK. They are people too and she is their family too. Of course they are going to ask.
I would reflect back on the questions they have asked, the things they clearly want to know, what you are prepared to share and open up a conversation with them. You can tell them that you understand they have questions but there is information it wouldn't be appropriate for them to have right now but you've given them something.
If you start a conversation with them intending to answer some of the questions they've asked before, with your narrative prepared, you are in control of it. Rather than feeling blindsided by their questions.
In terms of why we finally went nc, I've explained that I can't tell them that yet but I will be able to in the future and when I can, I will. I've also told them that they will understand both why it was necessary and why they couldn't be told sooner. They accept that because I've always communicated with them about it and they trust me.
Good luck.