A trip out locally taking 24 2,3 and 4 year olds, 7 staff and 12 parents on the bus?
I really don't think that the bus driver would allow that many people on, especially small children who will have to stand?
Buses here are always quite busy and I really doubt that it will go smoothly. The bus drivers also tend to pull away before everyone is seated/holding on and often brake sharply sending people flying.
It has been arranged with the best intentions and possibly would work well with a smaller group but with that many people I think it will be very difficult.