Well, I'm one who thinks able-bodied folks from preteens up should give up seats for those who need them. You know if you are not able-bodied. I know full well not everyone can stand in the bus. We always give up seats for someone who needs it, but it's not like it happens all the time. We try to go at slower times of the year so usually there are seats for everyone.
I totally disagree about not giving up a seat for a child. One time, my very slight DD who was about 5 yrs old had to stand up. There was nothing for her to hold onto very well. She was being thrown about while a family sat taking up space with packages nearby, refusing to get up. I finally had her sit on the floor and hold my legs as I was busy holding a 2 yr old and DH was standing holding a stroller. And no the bus did not look that crowded, especially since some folks were letting their packages sit. Also by the time the folks in front of us moved to the back, there were already people getting on behind us. For us to back off the bus (with 3 kids and a stroller) would have been hugely difficult at best. The good news is that after the first stop, an elderly gentleman and his wife (and I emphacize the term gentleman) got up at the front of the bus and insisted on ushering me and my DD and DS to their seats. He noticed from the front of the bus (we were nearer the back) that my DD was being thrown about.
I think for the sake of safety, small kids should not stand. The buses are not built for them to stand. Heck, I'm short and have a hard enough time myself. DH always gives up his seat to anyone needing it. Now my DD 10 also gives up her seat to those who need it, including smaller kids. My slight DD who is now 8 also gives up her seat as necessary.
I certainly do not think everyone should always give up seats for kids, but you can use good judgment. When you see a child who is too small to hold on safely having to stand, then get up. Common courtesy is getting lost. I always hold the door for people, men and women, and I will certainly give up a seat on a bus for those who seem to need it more than me.
It was only one trip that I saw folks being quite rude on the bus. Most of the time, I've seen very kind folks offering up seats to the elderly, pg women, parents with small kids, young kids. At WDW, I've seen less line cutting and other rude things than at our local 6 Flags.