@Sierra tango - it's not just the safety of your children, it's also the safety of everyone else. A small child landing on the back of one's neck could be quite damaging to *both* parties.
@ExXB - "Tall people have special needs [..] and they have to pay to ensure their needs are met " - a friend of mine who is *very* tall once mentioned in passing that he gets upgraded to first class when he presents himself at checkin. Because he simply isn't going to fit safely into an economy seat. But he is exceptionally tall - about seven foot. I've seen someone drop their pint out of surprise, just because he walked into the pub.
@RevMan2 - I like your style.
@GJP - it's quite difficult to reach across and fasten the seatbelt whilst staying strapped in. I don't think that a two year old can be "adequately supervised" from across the aisle.
@GrahamO - I quite agree. Others should not "give up their booked, paid for and reserved seats". The easiest way of ensuring this is to allow under-fives (and their carers) on as soon as possible. "We" (that is, responsible and considerate parents) do not wish to "freeload". I cannot talk for the irresponsible and inconsiderate.