"I don't understand why anybody would think mania, paranoia and psychosis wouldn't have the capcity to traumatise a child, unless they were not all that well aquainted with how this illness can present at the severe end of the spectrum."
No, that's not what I was saying. I was saying that it often is ignored. There was a major uproar on the mh boards the other week when I said I felt that a person self-harming when solely responsible for a child might be a safeguarding concern. Apparently it has "nothing" to do with child welfare. You will note that addiction and alcoholism are often frequently ignored wrt child welfare also, certainly they are in comparison with dv as though it's absolutely fine to live neglected in a house with someone unpredictable and unavailable but if they so much as raise a hand to you it's suddenly a wholly different kettle of fish. I see huge inconsistencies in response with some of my child clients really let down by services who minimise their suffering because they have parents "engaging" with mh teams or addiction teams (even though they are actually not currently able to provide best care for their children). I have no doubt there may be very heavy-handed responses in some areas and far too much leniency in others.
I personally had the experience of having my own mh interfere with my ability to provide optimal care for my second son. I was heavily engaged with mh services at the time and doing everything possible e.g. had been going to weekly CBT from when he was 10 days old, having CPN visits, all sorts... but communication, as Spero says, was appalling between teams. Early on I had a crisis about my son losing weight (breastfed) and my mh team decided it wasn't in my interests for me to weigh my son as I had OCD about it (and quite severely) and communicated this to the community team including midwifery and HV etc who basically totally "backed off" and ceased to even suggest he should be weighed.
In my anxious and depressed state, I totally misunderstood this as meaning there was no risk to my son, that the greatest risk was my anxiety. My husband was never informed nor was I that actually there was good reason to think that he wasn't thriving and that choosing not to weigh might impact on him. It was literally just left because the mh team decided, out of absolutely NOWHERE, that it would be "dealt with in CBT" and communicated this to the child health team (but never mentioned this to the CBT therapist, me or my husband). I never even realised they considered it was a key anxiety because I saw them telling the community team to back off as "justification" that weighing him was unnecessary as I was fixated on that at the time. Meanwhile, I was busily using my CBT sessions to deal with my fear of contamination on public transport and doing well with it.
Randomly when he was 20 weeks old, I went somewhere and a mother with a child the same age as mine made a comment to me about how skinny my baby was and suggested I have him weighed. I was much more stable at that point as had had five months of therapy and meds etc and it was a bit like the Emperor's New Clothes, I suddenly looked at him and saw that actually he was very thin looking. I went to the next baby clinic where it turned out he was on point of hospitalisation, having dropped from 91st centile to 0.4 of a centile. I totally FREAKED to the point I just shored up at MBU asking them to take him off me as I couldn't care for him and I had clearly totally failed. To this day I can't remember how I got from the doctor's surgery to MBU but I know I was not in my body and I can imagine that wildly wandering about with my son I was not very safe 
.
The CPN who I had trusted absolutely sat there and said to me: "well isn't it just as well you realised this on your own terms as if we had had to involve external agencies, it would have been very uncomfortable for you"....
They had NEVER EVER suggested to me there were ANY concerns about his weight to the extent I had complained about the midwifery team insisting on weights after their recommendation and had a written and verbal apology! I 100% believed what I was doing was in both our interests when actually, I was really not making a clear or rational decision and I was actually putting him at risk by not paying more attention to his feeding and weight.
It is a source of great pain to me, as someone who has been committed to child development professionally for my whole life, that my own mental health issues put my baby at risk at a time I genuinely believed I was doing everything necessary to protect us both. I think in many ways, the adult team just didn't see the risks to my son... they were so fixated on my wellbeing and on psychological risks.. the CPN had sat there challenging me in home sessions when I said he looked like he had no fat on his head because to her I was obsessing, meanwhile he was wasting away...
And so... despite what I said above... I do understand that adult mh services can be very adult-centric at the expense of a child's well-being and though my case was (in the grand scheme of things) very, very minor... and there were no long-term ill-effects, it was a sobering lesson to learn about miscommunication between adult and child teams. I still have to do a lot of meditation and work on it because the pain of that time is quite raw. There's something unbelievable and unspeakable about the idea that your mind could betray you so badly wrt caring for your own child... and I "only" had OCD/PND.
It's all a very thorny and sad area.