Can I just make the point that there is an underlying assumption in many posts that where this happens on charter and no-frills flights, people are getting a bargain and therefore should be treated in any way the airline sees fit?
Three things come to mind, and they all impact on the difficulties parents experience in sitting with their children on board as well.
First of all, in many airports, speedy boarders only get the right to board the bus to the plane a bit earlier than anyone else, which is about as useful as a chocolate teapot in assuring seats together on the plane. This is ripping off the speedy boarders, who might not know the arrangements at particular airports when deciding to stump up the charge.
Secondly, sometimes the speedy boarding queue is longer than the 'normal' queue, which again makes a bit of a mockery of the system and misleads passengers to a large extent - in Dublin once I got onto a plane ahead of people in the (longer) Speedy Boarding queue, for example, as the two queues were running concurrently! Again, this makes it hard for parents to accommodate their children's needs, even if they have paid.
Finally, no-frills routes are not always cheap - often there have been machinations behind the scenes to ensure they have effectively kicked national carriers off certain routes, and the prices are well in excess of what the scheduled tickets used to be, yet passengers have no choice at all if they need to travel on a particular route (Stansted to Munich springs to mind as one example). This is the capitalist way, but another way of looking at this is to say that air travel is a form of public transport (unless you have hired a private jet), and there are certain responsibilities that go with being a carrier (or indeed an airport) in relation to this, at least if you are using airports built by compulsory purchases of individuals' land, using public subsidy in many cases.
A way around all this is for the silly and somewhat artificial itemisation of bits and pieces essential to the journey to end, such as seating arrangements, credit card costs and so on. Passengers should be able to pay one price for a ticket, to include allocated seating and the costs of making the purchase, in the same way that compensation for delayed flights now has to be included according to EU law. I wonder if there is also a case for including baggage allowances once again, given that cabins are crammed with ridiculous amounts of hand luggage in the present system, that increases screening and scanning queues, waiting times and costs, and also runs the risk of falling out of overhead lockers in turbulent periods (I have seen this happen as well).
I think it's probably time to start asking for the service we want, rather than making do with the rather depressing service we are told we ought to be lucky to have. And I think we should remember that many, many people are making a lot of profit out of us in relation to flights, and our tickets could cover a lot more service than we are getting.