@Chickencuddle First of all, I’m very sorry for what happened to you, and I hope you realise how amazing you are to have overcome all of that to build a good life. You obviously have enormous strength of character and great resilience. I hope you take time to appreciate that in yourself.
I think maybe you’re dealing with two separate things here (and I don’t think you owe your father anything so I’d say put anything related to that aside. He has no power over you anymore - you have your own agency and don’t need to consider him)
First thing is about answering the question she has asked. As many previous posters have said, you can simply answer that you prefer not to discuss the details, but that you would not let your own children be in contact with him. She should be smart enough to understand what that means.
Second issue, and I’m trying to be very kind here, is your desire to have a relationship with her. I’d say based on her question that this might not be possible or what she’s asking for. So please don’t assume that you replying will result in that - that assumption will only lead to more pain for you. However people do mature with age and life stage so she might be in a different place - her latest message suggests some level of empathy. Just don’t hang your hat on this being the start of a new relationship with her.
I know this is really hard and painful but I think that the question that probably you might want be asking yourself is “am I able to help a mother protect her children based on what I know or will that come at such cost to my own mental health that I’m not able to engage in that?”
Sorry, probably a very clumsy way of saying that, forgive me if it’s not the right thing to say. But I personally wouldn’t judge you if you just can’t do that right now. You were taken into care so there were obviously concerns and she must know that children don’t get taken into care for nothing. As a PP said, there are various disclosures she could ask for from the police. The responsibility for this does not sit on you. The responsibility sits with the people who could have protected you, advocated for you, and not obfuscated or even lied about happened.
Whatever you decide, I think you sound like such a strong person who has overcome some much. Take a minute to be proud of who you are :)