I know you said you are going to take him but I still wanted to give MY oh so important opinion (total joke!!).
I would drive him based on the fact that they just don't seem to understand vehicle/child safety... even if you installed it there's no way to know that they tighten his harness properly or buckle him in properly...
Also, I don't know that he'd be going at all... where are they going? My heart starts to race just at the thought of taking 2 year olds into public without 1 adult to every 2 children. They are two, they are impulsive and you only have two hands. I wouldn't want to be there when one child needs a diaper change, another trips and scrapes his knee at the same time another sees something shiny, another grabs another by the shirt because he took her toy... I wouldn't want to have to help control all of that.
On a side note, an awesome field trip for 2 year olds would be Gymboree Music and Play, fully contained and their employees are in charge of entertainment so childcare staff can run crowd control. Hmm... I would LOVE to do a field trip there with smaller kiddos... and a van full of rear facing radians!