I'd ask them...they might just surprise you by telling you straight up! Or if they don't want to talk about it, then maybe a therapist.
My oldest who is currently 7, is the one I worry about most. I take her to a therapist. I was sure that she was going to be horribly traumatized and permanently scarred from the things that her father did while we were married.
The therapist told me that really, she's fine. I did my best to shelter her from what I could, but as everyone knows, no matter how much you try to hide things, they notice. Children are very perceptive. We still go to the therapist. Once every couple of months as just a check up really. That way communication is open and she feels that if she can't talk to me about something then, she can definitely talk to her therapist.
I also find that the children feed off of me. If I am calm and in control of the situation, they are at ease. If I am frantic and upset and STBXAH has me emotionally distraught, so are the kids.