Oh, Lisa,
How very, very awful. The teacher managed this just terribly. Whenever my son's class takes a field trip, lists are handed out days before as to which parent will be supervising which group of children. The teacher is very careful to ensure that no parent has more than one 'challenging' kid within their group (or their parent is asked to accompany the child for the trip). She's a pro and very organized.
I understand your concern and apprehension. I would speak directly with the principal right away, and make sure to do it in a calm manner. The school should have a written field trip policy for all teachers, perhaps depending on grade (for parent: child ratios) but some very clear protocol for handling any activities which are outside of school property. That teacher's philosophy that 'we'll all watch them' is *exactly* how students get lost. All the adults are counting on everyone else to keep track of the kids we can't see.
As a parent who has been on field trips, I can tell you that it's work enough keeping track of four kindergarteners. I would be distressed and frustrated as well.
Talk to the principal, be solution-oriented. Do not throw the teacher under the bus (I'm sure it's already been brought to the principal's attention and that they ARE upset about it.), but be clear that it was very, very upsetting for you as a parent, most especially for the child (I am sure her parents are furious) and that you would like to see a clear policy of assigning parents to specific kids in place for the future. Otherwise you can't, in good conscience, send your own child on the next field trip. Let them know that you would like to follow up on this, and if they don't follow up, do it yourself.
My guess is that this will make for positive change. Hard experience, so now they will have to learn from it.
ETA: I usually suggest talking to the teacher first, however, in this case A. this was a stupid, totally preventable mistake and teachers should KNOW this and B. this problem stems from a larger problem, being the lack of school policy.