I understand your fear in the first big field trip but....if you look at the time frame to drive to Milwaukee Zoo from McHenry vs the time frame to get to Brookfield/Lincoln Park Zoos I think you'll see Milwaukee is closer & faster to get to. For any of the 3 you would be on a expressway.....
The parent to student ratio is good. Like someone else stated if you know a parent who is going ask if your daughter can be in that parents group. This way you have their cell # and they can call you in an emergency or you could call for (ONE) quick update on how the trip is going.
They use these trips in lesson plans for the rest of the week....Math, Science, Social Studies/Health even English.
I know it's hard to trust your childs health/safety/well being to another parent. If you are really uncomfortable then you need to do what is right for you & your family. The one thing I would be careful of is transfering your fears on to your daughter....
When I was a school officer (SRO) I went on many of the field trips (w/2 of the schools) I had many parents tell me they felt better knowing a police officer was there "just in case". In 6 years there was never any major problems (1 bee sting lots of skinned knees) and the teachers are amazing....these events are all very well planned and the teachers have anything and everything with them~just in case.
We did trips up to Camp Timber Lee in WI(3 days 2 nights~NO parents) for 5th graders. There were also trips all over Chicago/burbs for other grades as well.
Good luck, know what ever you do it was the right choice for your family.