My response to that is......whoever BROUGHT grandma on the bus had better give up their seat for her.
Now before you think I'm dis'n you, I'm not. My 13 and 11 yr old kids are EXPECTED to give others a seat on the bus. They are the perfect ages to be the STANDEES. My 4yr old sits on my lap. He would love to stand too, but doesn't yet have the balance and agility skills for it (although said 13 & 11yr olds love to watch him try!)
We have been to WDW enough and have taught the older kids to automatically give up their seat for an elderly person, parent w/small child or stroller, or anyone that looks like they might just pass out from the day. If DH is holding the 4yr old, then I give up my seat too. I just feel better about having good manners than feeling all smug in my seat. I wish more people would feel that way.
There are plenty of people out there who could use more manners, the "with child" and the "child-free" alike. I've seen both sides of this on the bus. I don't like to see a 3 and a 4 yr old taking up a seat when mom and dad could be holding them on a crowded bus. I don't like to see a dad struggling to hold a toddler and lean on a stroller (just an unsafe situation for dad, child and whoever else is nearby if he loses his balance) when an able-bodied young person/adult is nearby and could give up his seat to them.
It's all just common courtesy and common sense while ridin' the bus!