What on earth are you freaking out over?
4th graders on a zoo trip...too old? Too young? I think we went on a few field trips around that time (Huntsville Space Center, some nature/science place in Atlanta, the Post Office, a local bird sanctuary, etc.) back in my schoolboy days (mid/late 70's).
Or am I not picking up on some sarcasm or something?
They were probably there to learn about animals, habitats, endangered species, global warming killing polar bears, inter-species racism and sexual identity issues, etc...you know, legit modern-day school stuff.
Nobody ever dreamed the kid was going to go and climb a fence into the croc pen (and then proceed to antagonize the crocs). That's usually not something most people think to do.