Good on you for taking a stand!

Firstly, there is NO way I would give up Charlotte's seat on a train for another totally competant adult - never.
She is just as entitled to ride in that seat as anyone else, she just gets to do it for free - doesnt make her any less entitled to a seat.

Secondly, I hope this doesnt ruin PT travel as a family for you!! We LOVE travelling by PT - Charlotte is currently obsessed with buses lol

My advice to you is if this happens again, do NOT give up Cs seat, stand your ground, and stand up for her rights