It seems that the parents were already Frustrated from the get go. I bet anything that they were already Frustrated and angry when they woke up from having to wake the kids up, get them dressed and to the airport. All this escalates into the resolution, which is to just get home.
Unfortunately, it seems that was not the case and everything that would go wrong went wrong for them.
I find it hilarious though, because they're assuming of "we'll figure it out" did figure itself out by them being the cause and them escalating it to the point where the staff figured it out for them and kicked them off the plane.
Now, I do wonder what occurred in the aftermath. If they continued their Frustrated anger to the police or if it escalated to the point where one of the parents got arrested.
This is what I mean with regards to people being Frustrated and responding to that frustration. These people expected others to feel sorry for them because they had kids, while disregarding the rules of flights.
Also, why didn't anyone else just get up and explain to them your solution of "One parent in the single seat and the other with the two kids." If someone had kindly spoken up and offered this advice. Maybe they would have realized, "Hey, that's a good idea". But since they were already in a frustrated state of mind, that idea and thought likely never occurred to them at all. All they wanted was to go home and expected others to feel sorry for them, without explaining why others should be accommodating to them.
This is a situation that has many factors to why it escalated and it's something that fascinates me due to how human behavior and response is in these situations. Like how people would rather not deal with it and let's the situation escalate to the point it did. Not only could the passengers offered up their seats by jumping up and saying, "Fuck it, you can my seats to be closer to your kids".
It's a two way situation and the outcome is dependent on everyone on that plane.
I've been on flights where some kids were sitting a few rows from their parents. And they would holler to them if they were okay. Or get up and check on them when we were able to get up from our seats.
Frustration and Anger are emotions that humans need to learn how to handle when situations do not turn out how one expects them to be. That is because the world does not cater to no one, and humans should not expect everything to go their way all the time.