I am also believeing the child who said their mother was locked in the bathroom, I cannot see how a child would even come up with this story unless something similar had been wittinessed in the past.
When DS was just short of 6 yrs he went for a day out with my sister & his cousins. Around lunchtime I got a very worried phone call from DSis saying how sorry she was about what happened with the plumber and was I ok, could she do anything, had we gone to the police? Etc.
DS had told an elaborate story about "the bad man" who came to "do things to the sink". Apparently I had taken him a cup of tea while he was working in the bathroom and he pushed me against the wall and wouldn't let go of me and "Daddy had to push him out of the door". The detail was incredible. What the man was wearing, how he'd banged my head on the wall and Mummy was crying afterwards, even how he'd left his tools behind and DS had played with them until Daddy took them away and put them in a cupboard 
We hadn't even had a plumber in, never mind all the rest. No one had attacked me. I hadn't bumped my head. I hadn't been crying. So it wasn't a semi-real story that he'd just embellished or got some of the details wrong - there was literally nothing in it that had actually happened. When asked, DS said he couldn't remember what he'd said or why. To this day, I have no idea where he got it from. Probably he saw a scene like that on TV, or else it might have happened to one of his friends who told him the story (he did tend to repeat his friends' stories as if they were his own).
Kids make up all kinds of stuff. Anything they've ever seen, read in a book, heard others talk about, can get stored away and regurgitated as if it really happened to them. One of DD's friends told me her brother had cancer, complete with detailed description of going to visit him in the children's hospital. Complete rubbish, we discovered later.
From what some pp have said, Chester zoo does have things that could pass for "tin hats" or "coin exhibits". And maybe the weather wasn't great and most of the animals were inside so he didn't see them - or maybe they just didn't make an impression on him. Not all kids go crazy over animals.
I'm on the fence (and shamelessly checking back for developments
) but frankly I'd be very wary of thinking the CM is lying just on the basis of what OP's DS has said. It's not a question of believing him - I'm sure he's not lying, it's just that what he's described could easily be consistent with a zoo visit, just filtered through the odd way that children often see things.
Either way, very awkward situation OP
. If you're really suspicious, is there any way you could loiter on the way to dropping him off until you see one of the other kids' parents approaching? Then you could get to the door at the same time and ask if their child enjoyed the zoo?