I don't see how YOU are the problem. You were asked to do them a favor. You have the right to say no.

When I was traveling in Europe, the train was crowded and there weren't two seats next to each other, so I asked someone if he would mind switching seats so my friend and I could sit together. He said no.

Unlike your situation, he would have been trading his window seat for an identical one two rows down. So it's not like I was asking him to sit in the middle seat...

well, I wasn't thrilled, but I didn't argue. Because he had the right to stay in his seat. I got out my laptop and stayed busy.

In my experience, most of the time strangers will accomdate you but if they don't? That is their right. No one is entitled to a particular seat. And in your situaton - absolutely the father could have paid an extra few bucks to guarantee the seat.