bcc...
I would suggest you reach out to a family counselor. Just go by yourself and explain what the kids are dealing with and ask for some parenting advice. They can give you great ideas for books to read and discuss, conversation starters, age appropriate ways to explain things... a good counselor can be a lifeline.
I left an abusive ex when my daughter was 4 and she never saw hitting - just heard yelling and breaking stuff... she was dealing with dad telling her lies like I cheated or never talked to him about what bothered me... My counselor helped me figure out what was OK to discuss with her, how to do it, movies we could watch and discuss that had parallel themes to our life and what kind of open-ended questions were good to ask.
Also things to do with her while talking - boys like to do something physical like throw a ball while talking about stuff. Girls tend to like to do things like play with dolls or do crafts while talking. Puzzles are good for both genders, all ages... Makes it easier for them to open up.
My relationship with my counselor was helpful as she got older and I had to deal with pre-teen stuff, too. Sometimes I'd see her every month. Sometimes once a year for something. But it can be an excellent resource when you need a little objective, professional guidance.
Start now - don't wait for the kids to start showing signs of distress. It's nearly too late by then as Elegirl has attested.