Not just from this thread, but I honestly don't think I've ever seen a site where the "its all about me and only me" attitude shows itself more. The parents being talked about in the past few pages work hard and use money that is just as good as any of ours to buy their tickets. The true problem lies with the airlines who chose to charge for anything but a middle seat, not the people who choose not to be suckered into that game. If anyone has a problem with the fact that they are stuck next to a 4 year-old then their problem is with the airline.
As you say the airline charges for anything but an aisle seat in certain circumstances -so if the family travelling is declining to pay to get seats together then it becomes their fault. Those extra charges are a part of the cost of travelling as a family. When my son was younger sitting together was more or less essential as he has mild autism and in an emergency would be more prone to panic-so if needed I paid for us to sit together. (Realistically it was never an issue -he's a great flyer just sleeps, reads or watches movies he was better behaved than many adults -but I paid to ensure if the unusual happened we were covered (it never did but hey).
I did at times choose airlines that offered pre-allocated seats at no charge over those who charged -other times I chose the better schedule/routing and took the hit for seat charges -It's a part of the cost of travelling with your children. If parents don't want to pay it and think they can scam a swop onboard that's down to them but to then get huffy with those who say no is the height of entitlement attitude and I don't believe parents who have chosen purely on financial grounds not to ensure they have their preferred seating are entitled to my sympathy or willingness to take a less desirable seat to accommode their frugality.