I'm not so sure I agree with the "it's unsafe for kids to stand on the bus" crowd. Under 5 maybe, but over 5.
The WDW buses are pretty much the same type of buses that are used here. (NYC transit) Kids here stand on crowded buses(and trains) everyday during rush hour.
Most of our students (over 1 million) do not ride school buses to school. The city provides them with metro cards so that they can use the city buses and trains.
Since I see kids 5-18 do this day in and day out, I'm not convinced it's a safety issue. If it was, I sure people would not be allowed to stand on buses. (charter and DME buses)
I don't have any stats to back this up. I'm just going by what I've experienced in NYC for the last 38 years.
I think parents should what they feel is right, and not worry about what others may think.