I don’t get the objections to interview by the family, is this because he hasn’t been charged yet and if he was they would have no option to object?
No idea, I'm afraid. Unless the children are in custody, which they clearly aren't, the police can interview them without parental consent provided an appropriate adult is present. I don't know why they haven't done so in this case. My guess would be that they thought they had enough evidence without interviewing the children and/or that, given their ages, their evidence would carry little weight, but that is just a guess. There is nothing in law that stops them interviewing the children.
Also do no cases run unless medical experts can say with certainty what caused a death?
Gross negligence manslaughter does not need certainty as to cause of death as here the prosecution only has to show that the defendant had a duty of care towards the deceased that they failed to fulfil, e.g. they failed to call for medical assistance when it was clearly needed. For other forms of manslaughter and murder, you need to show that the defendant's actions caused the victim's death beyond reasonable doubt. In most cases that is straightforward. It would be in this case if there hadn't been a pre-existing medical condition.
There must be Drs as expert witnesses who are called and give it could be evidence or no? It never happens
In most cases, there is no dispute as to how the deceased died. For example, if they died after being shot, there is unlikely to be any dispute as to the cause of death. Any dispute will be about who pulled the trigger. If there is a dispute, medical evidence will be needed. There is sometimes a situation where a medical expert for the prosecution gives one cause of death but one called by the defence disagrees and gives a different cause of death. It is then for the jury to decide which expert they believe (there is a serious question here as to whether juries are equipped to resolve such disputes, but that is how the system works). The experts must give their professional opinion and are supposed to be independent of whichever side called them. If an expert stated a cause of death for which there was no medical evidence, the judge would tell the jury to ignore their evidence. The expert concerned could end up being disqualified from giving expert evidence and could be struck off by the General Medical Council. The upshot of all this is that disputes about cause of death are rare and I would never expect a prosecution expert to say "could be" as that would immediately destroy the prosecution's case. I've rambled on a bit here, but I hope I've answered your question.