""Therefore the cost of them sitting next to an accompanying adult should be included in their ticket price. ""
The issue with that is, if you price up a holiday in the Internet and then go into Thompson/Thomas Cook (im quoting these because this is Who I've gone through), they can't always match the price, but they throw in extras and one of them is free, or reduced cost of pre check in bookable seats.
I always pay, or have the option, as said. My 18 year old DD doesn't like her LDs etc to be made public, so moving would get a flat "No", from us, as for children's noise, my eldest has ADHD and never shut up, I can drown anything out.
When I was a bit younger and holidaying in a group, our holiday started when we got to the airport and we'd have a drink on the plane, so I would have been pissed off at having to move, unless compensated by the airline.
I'm a LP, or rather was, on low wages, my holidays are the one thing that I spend money on.
As it stands, you need to figure that cost into the holiday, as a whole, if you want to sit together.