Well, I have recently had the same problem. My ex was verbally abusive among other things, but he was the father of my two young daughters. Good for you to show your children that isn't how people treat each other. You are very brave and should be proud of yourself. Keep up the good work! You are worth it.
But, I took my girls to therapy, and I also went to my own counselor, who started the D.V. rehab center in Salem. She is very well known and respected in her field and this is what she told me to say(along wth the counselor for the girls);"Daddy(step-daddy)couldn't be responsible with his behavior, so we had to leave. People who love each other show respect and caring and he didn't. It is my job to take good care of you and show you how to be responsible, so now we don't live with daddy anymore." Follow with the I love yous and will always take care of you.
Usually I find that satifies my oldest. But if she keeps asking, I just repeat it. They need not know details, just like you said, talking bad isn't a good thing, but giving them correct information is a good thing.
Good luck!