It's probably my post on the other thread you refer to.
I have no issue with children that are older and mature enough to sit properly on the school bus bench so that one of the safety feature of the school bus - compartmentalization, would work. This often means children older than 4-5 yrs old.
I don't feel that my 31 mos old toddler who is 28-29 lbs should be unharnessed on a school bus, kwim? But if the buses involved were the newer type with integrated harness seats, then I would send her on the field trip in one of the harnessed seats
Btw, I have 2 school-age children that have been riding the bus since they were 6 yrs old. So 5 yrs now for the oldest and 3 for the younger one. Our French Immersion starts in grade one and the children get bused then. Before Grade 1, they go to 'home' school and that often is a walk-to school. I felt fine when they started and they had orientation sessions to tell them what to do and how to sit, etc. on a bus.