On the bus just with my 4 year old. He likes to sit on those "high-up" seats behind the driver's cabin because he can see out of the window properly. The bus was pretty much empty so he sat himself in one of these two seats and I sat in the first "normal" seat after his seat.
About four stops later, two women get on the bus. One is about sixty-five, the other looks slightly younger. The elder of the two sat on the other high up seat next to my son, then her friend said to him "Can I ask you to move onto another seat please?" He looked a bit startled but got up and moved... then promptly burst out crying (probably tired as first week at school!) He didn't know why the woman had made him move from his seat and it really upset him. I thought she might have acknowledged me since she made my son move from his seat for some reason that I can't work out (these seats aren't disabled seats or designated for other passengers and since the bus was empty, the two women could have still sat together, just on another pair of seats)
My boy cried all the way home, the lady sat behind me made eye contact with me and told me that I shouldn't have let her dictate to my son about seating. 
DS has been asking why I let the lady be rude to him and why she was allowed to take his seat. I would use the paying child versus non-paying child argument but she had a bus pass, or the infirm and elderly argument (needing seat closer to door) but she was very able-bodied (had rambling clothes on and a huge rucksack)
Should I have stuck up for my son and let him stay sat there? He's really cross about it! He's not a brat btw, I would just imagine being sat staring out of a window minding your own then told to leave your seat is a bit weird when you're four!