It's funny how you don't think much of things like "bus etiquette" until you are on a board like dis. I didn't think much of it last year at the parks. The seven of us got on the bus, went to the park, had a great time, got on the bus, and rode back to the resort.
My father-in-law and I gave up our seats on occasion, but didn't go about looking to give them up. Sometimes our kids stood, sometimes they sat. If I was sitting, one of them was typically on my lap or standing in front of me. I don't remember getting any bad looks. In fact, I am more prone to give up my seat for a kid under 10 than an able bodied female. (no offense) Just think about how much those little legs have to move compared to us adults. My kids are in good shape, but were still pooped out and ready to sit after a long day at the parks.
I have mixed feelings about getting up for an able bodied female. My wife and mother-in-law always got seats before I or my FIL would take one. Pregnant? I would get up for sure. If a female has had a small child with her, I would probably stand up so that she can sit down with her little one in her lap.
However, if anyone goes on the bus expecting to have seats given up for them, that is just not right.
I honestly didn't see much of a problem on our 8 day trip last year.

I think some people like to make a bigger deal out of stuff like this than they should.
Dan