Wow, just caught up with you all. That's some pretty hard core WN ness going on. As I learnt from this thread, you can't play nicely with them. You can't expect a normal reaction, you can't behave towards them as you would others. Grey Rock and Ice Queen have been my weapons of choice!
Using the dc is, I agree, the absolute worst thing they can do. It makes you feel so impotent, and so conflicted as to what to say/do. There's no "right" answer, as if you let them see the real him it would break their hearts. If you lie and make excuses, it gives them the wrong messages as to how people should treat each other, and what they should accept from others. You can do the "they'll see what he's like when they're older." But that just sets them up for a massive shock down the line, maybe resenting you for not telling them more, and disappointment.
My approach - which only time will tell if it's worked, has been age appropriate truth. No slating or nastiness, but if I know it's behaviour they/I would condemn in others I condemn it in him. I've shielded them a lot from how I feel about the break up, but probably a bit too much. They got to a point where they thought only dad was sad, and deserving of sympathy. I'd suggest letting them see some of your sadness, frustration etc. But ultimately you want to be the strong one for them, basically the adult!
I try not to ask too much about what goes on between them and their dad. I try to keep it light and chatty, make their time with him positive. They see him regularly for decent periods of time, which makes things easier. If I strongly disagree with something he's said or done I try and talk about it there and then, not dwell on it. Once it's been said I try and put it out of my mind. However, I'm 2 years down the line, so I've had a lot of practice.
It is a tightrope. You want to protect them, preserve their relationship with their dad, be fair where possible and protect your own sanity. It's really hard, shame the answer isn't in the WN manual !