This situation offers some cautionary tales about how people view things from totally different perspectives. If a newborn baby was very ill in a NICU during COVID that is for sure an emergency situation where its father should be there. But - without excusing anyone’s behavior (let me say it again: without excusing anyone’s behavior), let’s take a look at how other people may have experienced the situation.
The 8 year old counts on seeing his father every weekend. He’s been told (or hopefully been told, if his parents have even the slightest clue about how kids think) that his dad will still love him and be there for him despite the arrival of the new baby. And now (remember he’s only 8 and thinks like an 8 year old) the new baby has only been here a few hours, and dad is already not coming to get him. (Probably, no one told the 8 year old how critical the situation was with the new baby either).
Th ex wife is likely fearful that once the new baby arrives, the father will lose interest in the 8 year old and begin making excuses not to see him. (It happens). And what she sees is, lo and behold, her fears are already coming true, right away. As an adult she should certainly be able to see the urgency of the NICU situation, but there is no other data yet to indicate that the dad will, indeed, continue behaving the same way towards the 8 year old. Maybe she does not know the seriousness of the baby’s condition either. She feels like she should hold a firm line and insist that the father keep his word about coming to get the son, because otherwise it sets a precedent for excuses and it’s a “give an inch, take a mile” type situation. Does that justify her insisting? No, but this fear could explain why she insisted.
It would have been wise for the dad to prep the 8 year old in advance for the chance of an emergency during the new baby’s delivery. And to have prepared an emergency plan for backup care of the son, one agreed to with the ex-wife in advance (so she would not have been able to insist the dad come get the 8 year old, having already agreed to the alternate emergency plan, even if that was something as simple as the kid staying with her that weekend). That way no one could have acted out of surprise from their fears.
The father could have explained to the 8 year old, “I expect to be coming for you this weekend as usual but sometimes things go wrong in the hospital when babies are born. Your mom and I already talked about this and if it happens, you’ll stay with your mom just until I can come get you, it may be a few days but I want you to know it’s only because it’s an emergency and I will be calling you and talking to you and missing you and will let you know when I will be there to get you.”
Thinking through another person’s perspective and potential fears means you can address them in advance. And then, those fears aren’t triggered and those people don’t react out of fear, because they have been reassured the thing they are fearing isn’t actually happening.
Is this annoying? To have to deal with other people’s fears, understand their perspectives, and pre-“manage” potentially damaging behavior? Yes, but it can result in a better outcome for you (and, you yourself have fears and likely sometimes react out of fears that you’d rather not have triggered, right?). It would have stopped the OP from developing so much resentment towards the 8 year old if the situation with the NICU and hospital hadn’t occurred. (That poor new baby is probably the subject of resentment, too, because of all the bad feelings generated).