My parents immigrated to Canada with me when I was 2. They left everything behind in China and literally started a life over from nothing. Just a few personal belongings, a few changes of clothes and a baby girl to raise.
Life was hard. And even at that age, I could tell. They had their moments of tension. It got minorly physical once (my Mom slapped my Dad on the arm), but more often than not, Nasty Verbal was as bad as it got.
They stayed together through it all and I have no doubt that their love for each other transcended anything that this world could throw at them. Those experiences and their example has made me what I am today. My husband and I have worked through issues that would have had other couples filing for divorce many times over.
The one thing that I wish happened differently though, is that they would have "debriefed" me directly, during or after their fights. I did get debriefing of sorts. When we were in our calmer, happier times, they would say things like, "yeah, we fight, but we always want what's best for the family/each other" and "we'll always make sure you're taken care of"... things like that.
I think it's true that kids know when something's wrong, even if you take precautions to not argue right in front of them. They can see the way you act around each other in the aftermath. They can see you ignoring each other or speaking in terse statements. So, if that's the case, I think they should be appraised of the situation, at a level that they can understand. Especially if the issue revolves around them (ie. you're arguing about after school care or extracurricular activity arrangements).
As they get older, you could tell them that "So&so and I both want to do what's best for you, but we just sort of disagree on what exactly 'best for you' means." Take this opportunity to explain the basics of discussion and debating to her - that if everybody takes turns saying what they think, then maybe someone will think of a really good idea that no one else thought of. And that way, you'll really find something that's the best idea.
That might take the edge off of the feeling that something bad is happening, she doesn't quite know what it is, but it's about her... I don't know if she's old enough to realize that, but as she gets older, she may start thinking that it's "her fault" that you guys are fighting.
BTW, I'm not a parent. But I've gathered a few things from my own experiences growing up, talking to people I know who are parents, and my Infant Psychology/Developmental Psychology courses. So whatever I say needs to be weighted as such =) No one else said anything about "keeping the kid in the loop", so I thought I'd bring it up. =)
ntang, if I'm completely off my rocker here, please let me know. =D