I thought I'd chime in. I'm a mom with two young children. The last time were in WDW was Dec 2008. I was pregnant and it was my son's fourth birthday. Before that we visited in May 2007, when my son was almost 2.5.
I have never expected anyone to give up a seat to either of us. However, on our last trip, there were many times when people saw us--a pregnant mom of a young kid and a dad loaded down with the stroller, bag of supplies, etc--and offered us a seat. Rarely in my life have I been so grateful for a gesture as when those people offered us those seats, and I was sure to let them know I was appreciative. Luckily, I was about 6 months pregnant and had just enough lap left that my son could sit on it so we could share a seat.
If we were to go to WDW today, the bus scenario would be something like this: husband handling stroller, diaper bag, etc., I would be holding my 15-month-old son, and my now 5yo son would be standing. Would I expect anyone to give up a seat for us? Absolutely not. But at the same time, I don't think the 5yo would have the stability to hang on without bumping into (maybe occasionally falling on) the person sitting nearest him. Since both my hands and my husband's hands would be full, we wouldn't be able to do much to help keep 5yo upright. Again, I do not expect you to give up your seat. Please, though don't get angry if the kiddo bumps you or falls on you at some point. It's not on purpose, and we do the best we can in the circumstances we find ourselves in.