Oh I don't know! I was hauled out of a bus seat once or twice by my grandmother as a child, to stand for an adult, I'm not sure about my parents, we were either on a train and the occasion didn't arise, or in a car. My thoughts are I'd give up my seat to whoever needed it the most, I could really have done with a seat once or twice when I was pregnant and commuting, not always visible until the end, I'd gladly give up my seat for a woman in such a situation, or someone disabled. If I had children or grandchildren with me then I'd possibly suggest that would be a kind thing to do. I think my sons would definitely do that and tell me they have done so happily when they felt someone needed a seat. One young man gave up his seat for me on the London underground and after I'd gratefully accepted my thoughts were "he thinks I'm old"
I did read the MN thread, I thought the mother sounded somewhat unreasonable.
Slightly off topic the people who annoy me the most on trains are the ones who "set up office" so they purloined the seat next to them and anyone who had the temerity to ask them to move their stuff they would often do so with really bad grace, huffing and puffing, I remember one such occasion I really wanted to ask "paid for two seats have you?" 
A famous matador gored by bull!
This made me quite teary - but smile too

