'We should model what we want to see - that means courtesy and thoughtfulness to kids.'
I agree with this part. Adults need to set good example too, by offering seats to those less able to stand. But I don't agree with the 'kids come first' or 'kids are weak and delicate' attitude. Most kids do put themselves first until reminded to think of others.
I disagree with poster who said 'children are less strong' and need the seat more. How is a healthy child less strong in a way that limits ability to stand? I appreciate they may have less stamina and less body mass (eg to lift heavy items), but they are perfectly capable of standing up. And they are far more flexible, with more energy than the average adult. The likelihood of a child having joint problems, foot problems, pain/stiffness in legs, low/high BP, shortness of breath, balance problems, being laden down with heavy bags, having been on feet all day etc etc... is far less likely. In contrast many adults (particularly those over 40) have chronic health conditions that make it hard to stand for long periods.
I know plenty of women (fit healthy women in 30s like myself) who feel faint standing on stuffy trains for long periods. Particularly when they have their period, or are pregnant, or have been too busy to eat for a while. It's very embarrassing to ask for a seat because you feel faint, sometimes people pass out before they ask someone to move a child. How many children get faint or dizzy from standing? How many kids have exhausting days at work where they are rushing around all day, followed by a supermarket dash before train? If more children sat on laps or stood to let ladies sit down (as they did when I was young) people feeling faint would be less of an issue.
If child can't sit on lap why can't they sit on floor, perch on a luggage-rack or stand in aisle? There is rarely space for adults to do this but kids take up less space. Would you really let a woman in her 50s sit on floor in aisle because your 5-year old 'needs' the seat more??
It seems many parents think their child's 'need' for a seat overrides manners and respect for elders. How many kids are so fragile, delicate or weak they cannot stand up for a while or sit on floor? Maybe they spend too much time being sedentary in front of tv so genuinely lack energy or inclination to stand.
Referring to other posters comment about sitting on flood post-c-sect because kids took up all seats, I think it's appalling kids were allowed to take up seats while adults sat on floor! Lots of adults (not only post-surgery) find sitting on floor painful and difficult. Why on earth didn't parents prompt them to move?