I also think that it depends how the "tension" plays out. Do mom and dad fight in front of people? Do they give each other the silent treatment? If it is that blatant then mom should get the choice - he gets left off the guest list or they all do. She can decide which is ess damaging to her kids. If it is more subtle and the family can ignore it then maybe they can tolerate it, at least for now.
This is exactly what I was thinking. If there is conspicuous public conflict between the couple, in other words, if they couldn't put their differences aside long enough to behave in front of others, then I would hesitate to invite them both to anything. But honestly, that might hold true even if they weren't getting a divorce.
On the other hand, if it's just tension but the couple is conducting themselves well, then I'd go ahead and respect my relative's wishes and invite them both.
Regarding whether the kids know, it's almost certain they have some inkling of what's going on whether the parents have told them or not. Kids, even young kids, are much more in tune to those kinds of things than we might imagine. On top of that, if they've told other people, the chances that someone will let something slip are enormous so the kids will find out that way if they don't already know.
Remember, kids that age have no idea what is considered "normal". To them, "normal" is pretty much what they've experienced. So even if they know their parents are splitting, it might seem normal that they are still living as a family, that they are all invited to family events, etc. because they really have nothing else to compare this experience to.
So in conclusion, I'd say, unless the couple is disruptive in public situations, I'd invite them all.