We were in WDW last week during the busy Thanksgiving holiday. Several times on the way to or from the MK the buses were packed. But we got on, with me holding DS (3 years old, special needs so a bit tippy), and I was totally fine with standing: I can hold on with one arm and hold him with the other, or let him stand and hold onto him with one arm. Never once did I get the opportunity: EVERY SINGLE TIME at least one person immediately stood when I got on the bus and offered me their seat. Actually, usually it was several people including a few teenaged boys.
One boy was obviously hoping someone would take his seat, he kept offering and it took him several people to find one who would accept it!
Maybe I'm just lucky, but everyone on the buses is extremely courteous and kind to us. And when I accept the offer of a seat, I put DS on my lap.