But anything less than about 6 years old, if travelling in an airliner, should be secured in a soundproof, leakproof container somewhere in the cargo area....
I'd have to take issue with that. Have flown on airliners a fair amount with my son as his mum is a freelance musician and he's still breastfeeding. Also a trip to Australia to meet ailing grandparents for the first time. He's fairly relaxed about it - but 40 hours door to door for the Australia trip and I think any toddler can be excused a tantrum or two.
Not necessarily much fun for anyone, parents or bystanders, but part of that live-and-let-live thing.
German trains often have parent-and-child compartments which I like, but can't see as being practical on airliners.