What is the norm with the other caregivers in terms of outings and what do the parents consider outings? For some a trip to the park is an outing and for others it means a trip to a museum or music concert. I would lay it all out and include in the cost of trips the insurance on the van, gas etc with the idea if you weren't doing field trip you wouldn't need that coverage and do your best to convince families that economically it is just not viable for you or the parents as you can no longer absorb the cost - any of it.

Also first my sympathies go out to everyone involved in today's bus/train collision but because it happened so close to home for you I would be taking a firm stand and saying to parents enough is enough I can not take on the liability of having your child out and about. I could not live with myself if anything happened to them on my watch. Almost everyone who lives in Ontario has seen via rail go by on a level crossing and had to stop for a train or taken their charges on a city bus thinking it was the safest way to travel.