I agree with FIB, you have to really try to not say anything negative about your wayward wife around the kids. The kids will come to their own conclusions over time.
Here is an excellent article about divorced parenting mistakes:
I had a difficult question posed to me last week by my much younger kids - "Daddy, why dont you want to meet and be friends with Ed (OM)?". The only quick answer I could come up with was that they would understand when they got older. My inner thoughts wanted to say "because he is a dishonorable wife stealing scumbag!"
