@Yellowlegobrick
HDU is often cubicled. ICU environments are mostly not, for good reasons (which I'll come to).
So we are are talking 8+ critically or at least seriously unwell babies in one room with constant alarms. Parents and babies who have a need for privacy and dignity at critical moments (which may require the other parents to leave the room). Staff who need to perform safely and effectively whilst they undertake technically challenging high stakes procedures, and could not reasonably do or even have space to move if being being scrutinised by 8+ sets of parents. Parents could get no sleep at all, having parent beds would be almost pointless. Babies are in these nurseries sometimes for weeks-months.
So there's no way parents could sleep next to their babies in a non-partitioned (eg open plan) NICU setting (other than in the low intensity feeding, growing and soon to go home nurseries - in which case there is already a big drive for this but it is limited by the need to overhaul the physical layout of units / space available which often isn't logistically possible). Even if it were logistically possible to introduce partitioned ICU spaces (eg cubicles) to allow 24/7 parent presence with facilities for sleep - this approach would come with inherent safety risks in an ICU setting (reduced oversight of critically unwell patients, difficulty re where to keep lifesaving equipment - the more duplicates of critical equipment there are the more likely a stock or settings error, but you ideally want this equipment immediately to hand and in the same room as critically unwell patients).
Also, the approach of partitioning medical spaces could counterproductively increase the risk of malicious harm by increasing the opportunities for perpetrators to be alone and unwitnessed with a patient.
The truth is that injecting air into an IV line is easy and takes seconds. You could probably do it whilst a parent was awake at the bedside and they may not be aware. You could easily do it whilst they were asleep.
The only thing I can think of on a systems level to tackle the issue of medical serial killers is to embed a formalised process of asking "could this be a serial killer?" into the SI review process. Every single unexpected death is investigated but the question of whether it is a serial killer is not likely to be asked. But there are risks also to this approach - the possible effects on staff morale, the possible effects on staff relationships (because generally, respect, empathy and trust make teams more effective at keeping patients safe but these are effectively barriers to identifying a colleague as a serial killer), the possible room for abuse (imagine such an investigative approach being weaponised against a whistleblower for example), the possible damage to systems focused culture (which is more likely to result in system change and improved patient safety vs individual blame culture which makes people afraid to reflect honestly on mistakes and why they occur, and ultimately reduces patient safety). So it's a quandary. I guess maybe there could be national mandatory safeguarding guidance on when to involve the police (or potentially some specialised expert professional whose entire job is to look for evidence of medical serial killers?) - eg if insulin OD is suggested or plausible, if unexpected deaths cross a certain threshold, if a member of staff feels concerned about it, if there is an identifiable but unexplained pattern of some kind to the deaths. I think it is very hard to systemically detect/prevent something so rare and so different to other patient safety hazards (and the overzealous pursuit of which could potentially increase those other less dramatic but more impactful hazards).
Sorry didn't mean to ramble on.