Kungfupannda - you and I would have a great time together on the train!! Whilst at university, I once got on the train at Crewe, travelling to London to visit dh (he was dfiance at that point). I had booked a window seat at a table for four, and when I found my seat, it was occupied, but the aisle seat next to it was free so, rather than making a fuss, I started to sit down there - at which point the party of three business types occupying the rest of the 4 seats informed me that the seat I was taking was booked by them. I responded that, in that case, I would have MY booked window seat please, and looked firmly at the chap in my seat until he meekly moved so I could sit down.
The best part was that, over the course of the journey, I gathered from their conversation that they'd been at a training course in Crewe. The subject of the course - Assertiveness Training! I don't think it had taken very well! 
I have to say that I absolutely agree that it is the airlines who are creating this problem, by their policy of charging for optional extras - some of which are clearly NOT optional for some people. I also think that airlines who have this policy should make it absolutely clear in all their advertising etc that you will not be guaranteed seats together unless you pay for them, so that people are aware of the full cost of what they want long before they book it, so that it is part of the budgeting process for the holiday. This is what dh and I do when we are planning a holiday.
However, like many on this thread, I think that the policy needs changing, as it does look like blackmail on parents (or anyone who needs to sit together - a disabled person and their carer, for example). Perhaps the easiest compromise would be to allow only a certain percentage of seats on the aircraft to be prebooked (or only sell a certain number of priority boarding passes), so that families who didn't want to pay for the guaranteed seats together/priority boarding, would at least have some assurance that getting to check-in early should allow them seats together.
But I do not like the attitude that people without children must give up their prebooked seats so a parent can sit with their child, otherwise the parent will happily let their child make the flight a misery for the people they are sitting with; or that parents can 'demand' that the childless person moves and can call them names if they don't (even if that name calling is here, anonymously on the interweb). I've seen an attitude of 'I don't care if they've paid, they have to give up their seat for MEEEE and my precious child' that is not pleasant, and would get my back up (but would NOT get my bum up off the seat) if I was faced with it on a plane. Guaranteed seats together cost money - manners don't.