It's horrible you've been through all that.
I don't think that you are being U at all.
A (male) school friend of mine went through similar childhood experiences with his mother, and was understandably NC. When he heard via friends/relatives that she was attempting to become a foster parent (in a different city), he felt he had to take action to prevent (already vulnerable) children from harm.
He immediately drove four hours to the council offices in that city and asked to see someone urgently, said he would wait as long as it took.
Whilst he was waiting he wrote a bullet point account of what had happened to him and his siblings. He said bullet points helped him put in in as detached and brief a way as possible.
And he did see someone, and they did read his account. And they checked the records under his mum's previous name in s different council and she was not allowed to foster.
I relate that account because I think it might be an idea to a) write down what your children may need to know and b) get someone else to at least look at it. It would be better if someone (counsellor, friend) could be more involved in helping you communicate what happened to you.
It will still be their decision, but I think it would've best for them if it was an informed decision. I also think you need help in formulating and delivering that information to them in a way that is both truthful and least damaging to them.
She won't be able to steal your children. 