My husband and I have shuffled our seats around many times, most often
without being asked. We've done it so couples can sit together. So women in hijab can sit next to me, instead of my husband. So parents can sit next to their kids (that last is a bit selfish on my part - I don't want to have to entertain/console/care for the kid!). I'll take a seat next to someone obese, so my husband doesn't have to, since I'm the skinny one in our family. Would I move in the OP's story? I really don't know! How rude is "rude"? I'd probably have asked the child where their parent was when I sat down, and things would have unfolded from there in a very different fashion.
As for another person's story about being forced to sit next to the family from hell... when I found out that dad was back a few rows, I think I'd have insisted he take the seat next to his family. And I'd have tried to get the stewardess to back me up. At the very least, I'd have made sure he understood what a sad, selfish excuse for a parent he was.
(Yeah, I'm friendly and accommodating, but I can also get quite snippy when my back's up.)
I find it very disappointing that the OP never returned to clarify her original post. S/he just tossed it out there for everyone to argue over. We don't know how old the child was. We don't know whether the seat she'd have been trading for was also an aisle seat. We don't know anything.
All I do know is that I'm not particularly possessive about my seat. I just want to get to my destination, preferably without a child screaming in my ear.