Hi WOL, well, I'm not the best person to suggest what help might be out there because I didn't have any but it sounds as though something for you and something for the DCs could be useful. There will be charities e.g. Rethink who'd be worth a call and may be support groups / networks. Your GP may be able to advise and offer counselling, or CAB but I'd search the web too.
It sounds as though your husband is starting to be able to talk about the difference between the depression and the histrionic behaviour. That's important because, as you've discussed above, the manipulation is not necessarily part of the depression and shouldn't be wrongly portrayed as such to the DCs. He needs to think about what he is imposing on them, as well as you, recognise that this needs to be explained to them, and seek diagnosis / treatment for these problems. I suspect there may be help available that could make a lot of difference, coping strategies etc, if he wants to stop doing this.
From my recollected child's perspective, I'd say that what the DCs need most is some basic information, which gives them a source and contact to come back to and add to later, if they want. They need to know there is a problem, that it has a cause (they may be concerned about inheritance) and that steps are being taken to address it as far as possible and to manage what remains. They probably won't want lots of detail, lengthy discussions or to feel burdened by the whole thing as they could perceive that as more of an imposition on their lives than the illness itself. They musn't be made to feel reponsible for looking after their father. It's also important to understand that, while everyone else can be as supportive as possible, responsibility for managing his own illness lies ultimately with your husband. That's a harsh truth - if afflicted with misfortune you have to deal with it, however unfair - but it is true (in life of course, not just illness).
On the suicide risk, I don't think I'd tell them about the threats. That's partly because they are threats, to you, and part of the histrionic / manipulative behaviour which he can choose to address, so they could diminish or disappear.
That doesn't mean he will necessarily stop being a suicide risk, though I'd hope your sense of the likelihood of this will, reasonably, diminish. As far as the DCs are concerned it's probably enough to know that depression comes with a risk of suicide (I don't know about personality disorders), though this depends a lot on its severity and good management.
This may seem a grim thing to say, I hope it seems worth mentioning, as an aside, not intended to be at all fatalistic. When my mother died I found this much easier to accept and deal with because I'd long known it was possible. It was a shock but not a surprise. I was angry about circumstances which made it preventable but also recognised that was only true until the next time she was very ill, or the time after that. I didn't experience the 'disbelief / denial' stage of grief because I did believe it was real and had happened. I also didnt feel any sense of guilt or responsibility, something people usually do with suicide (even the most distantly associated people, who wonder if they could have 'done something', usually because they mistake suicide for a rational decision and have least awareness of the illness). I see my mother dying from her illlness as little different from someone with cancer dying from that.
On a more positive note, I know lots of people who have or have had depression. They've used a variety of therapies, drugs and the natural remedies - exercise, good diet, sleep, friendships, solid relationships. The ones with reactive depression (to childhood circumstances) have largely got past it and others have found good management strategies and prevented further episodes or improved over time too.