I understand how you feel as well. My DS2 is 5 and in Yr R and going off to the zoo in a couple of weeks. He is soooo looking forward to it but I am a little apprehensive only because he hasn't been on a big trip without us.
However, whilst I have reservation because he is so young I also know that the school have done this trip every year for the past 5 yrs to my knowledge, probably longer. They know exactly what to do, they have it all planned out. No school, I would hope, would embark on this sort of trip without first having planned it completely, including a pre-visit visit to the place by some of the teachers.
Probably like my DS your DD will have been studying animals as a topic in a run up to the trip. They will be told all about it and they will be told exactly how to behave and where to go for help. They won't be left alone at all.
I know it is easy for me to say as DS1 went on this trip at 4yrs and he was fine but really, there is nothing to worry about, just think of it as a slightly longer than normal school day.
My DS1 is now 8, nearly 9 and has been on school trips every year since and has loved them all and not once come to any harm.
I think if you were seriously considering it (and I know you weren't really) not letting your DD go on a trip until then would be doing her a great disservice - they really are part of growing up. I can still remember every single one of mine even the slightly dull ones.
Just to put it in perspective slightly, never mind holding off on day trips until 8 or 9, nowadays it is quite common for them to go on 2 or 3 night residental courses at that age. That is scary!