My immediate feeling is that the airline restricting the sale of tickets to children under the age of twelve is actuated by sheer laziness.
Their cabin personnel should be trained to deal tactfully with children whose parents are ignoring bad behaviour and to deal firmly with the parents who let children kick other passengers' seats,
Crying babies is a different matter - the change in air pressure inside a plane affects infants' ears, Sometimes rubbing the child's neck just below the ears very gently can help. The flight attendents ought to know how to do this and how to demonstrate it to the infant's parents.
Might it not be possible to sell seats to travellers with young children - infants to 10 or 12 say in the same part of the plane, so everyone there was so to speak in the same boat?
Most parents do still dread their children being seen as annoying, and anyone who has tried to stop an infant in arms from wailing all night, knows how embarrassing this is anywhere outside your own home.
Ear plugs are advisable on any long night time journey, and telling a child who is old enough to kick or push your seat firmly to desist does work you know. And if the accompanying parent gives you a mouthful of abuse or sleeps through it all, tell him or her firmly that it is their job to prevent their children making pests of themselves.