Do the right thing, move your ass!
Sometimes the cost to pick my seat on a lane is almost a hundred pounds. Imagine that with a family of four.
Airlines charge this because they can.
Sometimes the flights I am on have barely any passengers. But you are still paying for extra legroom, or an aisle. And they want you to pay FOUR TIMES to sit together.
When you have extremely small children, they need their parents close by. They just do and honestly, you WANT those parents close by.
I refuse to pay a seat upgrade charge. When I fly in April it would be three lots of extra charge each way it could be almost as much as my entire flight. Times FOUR as a family.
So no
If a parent of young children needs me to move, Ill move. And you can sure as hell know I wont make them feel guilty. A couple need me to move so they can sit together and Ill move. AGAIN, if they aren't able to pay a prohibitive seat reservation charge that from my experience is rarely necessary, I will do my bit for the common good and shift my ass.
Your mileage and monetary liquidity may vary. But having been a parent of young children flying, IF you hold your ground, I hope those kids wail and kick your seat for every minute of those 21 long haul hours.