"most of the time I am happy to put myself out for others. There's the odd occasion where I'm not but I'd extend that to 90% of the time."
On the train thread, TheCatsMeow, you stated you'd leave your buggy in the vestibule of a train, if you got a seat and if people had to climb over it, that would be okay by YOU - not a shred of realisation of how hazardous that would be, for people trying to move through the vestibule! How the hell is that 'putting yourself out for others'?
Clue - it's not - it is lazy, entitled selfishness.
If the mother had waited until the person who'd booked the window seat got there, and had asked nicely if they would let her dd use the seat (for all or part of the flight), then I'd say the OP should at least have considered her request - but plonking her child in the seat without asking was entitled and bad mannered, and a very poor example to her child!
If she was so keen for her dd to have a window seat, she should have paid to book one, instead of nicking someone else's!
If she was in a restaurant, and someone had the last portion of the dessert her dd wanted, would she waltz over and nick it from the other diner? Because, as the OP's employers had paid extra to book that seat, that is pretty much what she did. I bet Meow would expect the other diner to hand their pudding over for little Audacity, though - for Audacity's joy, natch.