I've seen it all.
We have a son with a rare medical condition, and one night when he was 5, he wasn't feeling well, so I was taking him back to the resort while the rest of the family stayed at Magic Kingdom. We had to stand in the front of the bus. He kept falling asleep and I tried to hold him, but he was too heavy. (I was also balancing a diaper size bag with his medical supplies.) When I tried to stand him up, he kept falling over. I apologized as he slumped this way or that way, but no one said a word to us... not even a "poor little guy is pooped right out," or an "awe," etc. This bus had the 3, or so, seats that face each other in the front of the bus. For the ENTIRE ride, the childless adults in this party, just stared at us!
For one man in particular, it felt like he was being openly defiant... as he sat rather slouched, knees spread far apart, arms folded, and stared as I struggled with my son. Finally, the bus driver said to sit him on the floor, leaning him up against
her seat. I said I didn't think this was allowed and the driver said he would be alright. As I bent to put him in place, she said she was sorry some people were so inconsiderate. With my son on the floor, I tried to straddled him with my legs to try to "hold" him in place with my legs, while I held onto the poles with my hands. At times, I squatted to hold him, (my knees were killing me.)
These people didn't need to give up their seat for us, but would it have been too much for them to squish in and make a little room for a 5 year old? Really? Could one of them offered to just hold my bag while I attended to my son?
All of my children who are old enough to balance on a moving vehicle, know to give up their seat for the very young, the elderly and parents holding children. And, even if we are among the first people on the bus, my DH doesn't take a seat because he knows someone will need that seat more than he does.
When standing myself, I've held onto strollers for strangers, so they could better hold onto their child because no one offered to give up their seat for them. We've even put siblings on siblings laps, to make room for another "tiny hiney" on the seat.
I know everyone is tired... but common sense and common courtesy isn't too much to ask for.