Hmmmm, I'm on the fence about this. She did reserve the seats. I'm glad that the family were re-seated in First Class.
But -
If the couple were elderly, they are less able to stand than 12 year old children. And given the "sad face" Daily Mail story, it is unlikely that this mother is teaching her DC to think of others before themselves (particularly given that she thinks that a "posh" voice is a moral failing).
It's a difficult situation. An on-the-ball train manager should have dealt with it, but usually there's only one on a 6 to 8 carriage train.