Welcome, Lala7! This forum is a real help.
I resonate with what you write, particularly:
I'm struggling with how to validate their experience without bashing him.
I'm not sure the best approach but here's what I'm doing. When my children say something about my uBPD/NPD co-parent (we are separated and divorcing) like "he gets angry at us over nothing" the first thing I try to do is notice how this makes me feel. I feel validated because this is my exact experience. I feel sad for them that they have to experience this too. I feel anger at my co-parent. I feel some guilt that I get to leave the relationship but the children have to live part-time with him. I feel fear that the children are not equipped to deal with his illness and it will harm them or they will take it on themselves or they will become alienated from me. I feel relief, pleasure, and vindication--sweet vindication!--that they share difficult things with me. It is amazing how a child's comment, sometimes just made in passing, can bring up so many emotions and thoughts for me.
How to respond? I'd love to hear what others say. It feels essential that I validate the children. Lord, do I know what it is like to try to tell someone my husband is an angry person and they tell me how affable and kind he is to them. And yet, with our children, we know how important it is not to bash our co-parent--we know this both intellectually and morally and also from the experience of being bashed.
I've been responding "I'm sorry he got angry with you" and "oh sweetie, I've experienced that and it is really hard." I do not say I've experienced it
with their dad but keep it more vague. I really have to hold my tongue to keep from saying more.
Depending on the situation, I might ask the children a question such as "How did it make you feel?" or "How did you respond?"
I might talk about how I feel when someone is angry with me and ways (both that I'm proud of or regret) that I responded.
I will talk in generalities and use myself, not my co-parent, as the example. My children complain that their dad brings problems to them they can do nothing about: parentification. I might say, "Yes, I've experienced this too and it is no fun. All adults, just like all children, have problems. That's normal. And we all need to talk about our problems with others. For adults, it's important that we take our problems to other adults. Who do you think I go to with my problems?"
Finally, there are times when I don't know what to do and I'm increasingly saying this to my children: "I'm struggling to know what the right thing to do here is." And because two of my three children are in counseling there are also times I say, "Why don't you bring that to Therapist? I think s/he is better suited than me to talk about that." Many times I've told my children therapists are both neutral parties and professionals trained to deal with these very issues. That's why we use them.