I STRONGLY suspect we are not getting the whole story here. TWO ultra-rude people on the same train acting like this on the same day? I theorize that the author(s) had a "tone" in their voice when wandering around/demanding seats together. I have NEVER seen anyone refuse to move to allow someone to sit next to them, and here on the corridor, you hardly EVER sit alone for long. Not to say it doesn't happen, but it's surely unusual.
I'm also not playing holier-than-thou, here. On a recent Acela trip, I may have had a bit of a "tone" with the conductor when I started seriously doubting there were ANY seats available on the whole train (the few seats without seat checks had people sitting in them). The conductor had to move someone to get me ANY seat, much less a seat with the person I was traveling with (facing backwards, not fully reclining, and at a table with no legroom or footrest, besides).
In any case, I tend to agree with an earlier poster: I enjoy my window seat greatly. I didn't catch how young these children were, but unless they were under 10, they should have been fine sitting alone for a few stops. I'm not saying I wouldn't have moved for a family, but they would have had to be VERY nice about it, and made it clear I was doing them a favor, not just "my civic duty."
Contrary to popular belief, people DO ride trains . . . and that means seating arrangements won't generally be ideal.
JPS