When I played TS2, I noticed the children would cry when in the presence of a cheating parent. I wasn't sure why they were doing this. So I googled it and found out they were hurting due to one of their parents being unfaithful. I felt terrible. LOL the sim that cheated had the trait of wanting a lot of lovers. (I forget what it was called).
I haven't seen it happen in TS3, because I don't really allow my sims to cheat. UNLESS I am playing a premade sim and find out the husband cheated. I know it isn't right, but I allowed the sim getting cheating on to cheat as well. I didn't notice the kids behaving any different, it was like they didn't know.
someone above stated their sims kids didn't like the new step mom. How do you know? These sims don't seem as emotional as the ones in TS2.