^^^ and major karma points to your kid!

It is entirely possible, and plausible, that the child in question had disabilities beyond the physical. Perhaps mental or emotional impairment that could to some degree be soothed by having BOTH parents within eyesight during a potentially stressful and confusing experience like a flight.

I am disabled (minor to me, but then I was born with it. Others view my congenital birth defect as being pretty significant). I have yet to be on any, repeat any intracontinental flight within NA or Europe where I truly cared about my seat. Except middle seats. Hate middle seats in Y. Now, on TATL or TPAC flights, it is another deal entirely. I have extremely specific seat needs that just cannot be reliably dealt with via a travel agent or even airline staff. I have yet to deal with anyone while booking who can definitively answer extremely specific hardware question like "are the seat controls on the right or left on this aircraft?" Once I am familiar with a particular aircraft typically used by airline XYZ on a given T-oceanic route, I can deal with it. However, there are times I must request a change after boarding. I will usually ask a pax in a "desirable seat" nearby, and offer an explanation. Sometimes the pax responds viscerally, at which time I apologize and get an FA involved to help without impacting the disturbed pax. In those cases, if the exchange has been overheard by others nearby, the visceral pax usually starts getting a tongue lashing from others, WHICH I UNIVERSALLY PUT TO A STOP. I have always been able to reasonbly re-seat. The disturbed pax usually ends up apologizing, 9 times out of 10.

Now, on the other hand, I have also been asked to move by FAs to accomodate others. Sometimes not even asked, just unilaterally moved. I generally accede, but do what the aggrieved gentleman in the OP failed to do. I clearly and concisely state my requirement for my new seat to the FA, and make sure they understand why said requirements must be met. This has on occasion created a chain reaction of shifted pax, but if it truly is necessary to accomodate a realistically needy party (the one that bumped me), then the FA gets it done. If the aggrieved pax in the OP (or other posters to this thread for that matter) truly NEED an aisle seat, politely stand your ground while being reasonably willing to help an obviously needful pax. There's no evil in spreading the pain to others in the cabin if it helps someone less fortunate.

All that being said, if it's just because two buddies want to sit together, then they can pucker up and decide which of my nether cheeks to kiss.

Finally. To all who think they are too important to help a disabled kid (save those who have their own private disabilities, which could be dealt with as described above), shame on you. Especially for a measly NA transcon. Trust me, the 4 hours of pique and potential discomfort you might face pale in comparison to the lifetime that kid faces. I certainly hope nobody you care for is ever stricken so. Instead of thinking how important you are for once, think of how a relatively minor sacrifice on your part might be the best part of someone else's day, or week, or month, or year. It does eventuaally pay back, either way.