I got the same kind of response a couple years ago when I had a "quiet word" with a pet shop manager, who proceeded very loudly in the shop to say I didn't know what I was talking about, she'd kept rabbits and guineas together for years with no problem, yada yada yada. Can't say I'm sorry to say the pet shop is going out of business. Karma! :)
I know people will cite examples where they've had rabbits and guineas living "happily" together (as if they see what they are doing 24/7!) but the risks are just too, too, high. I've seen the damage that can be caused to both - including a rabbit with severe intestinal damage from a too veggie rich diet (which the pig needs but she didn't) and severe injuries caused to piggies by rabbits.
I will just about accept that if you have tonnes of space, the pig might have a chance of escaping attacks by the rabbit. However, people visiting petting zoos and such like will see rabbits and guineas together and assume this is fine and dandy, so will do the same at home, with usually tragic consequences.
I'm guessing the city farm wants to pack in as many different types of animals so the children can have a more "enriching" experience, and if they don't have space, they want to double up the pigs and buns. It's setting a terrible example and is ethically irresponsible.
Pet shops want to make a profit, and think the "offer" of two different animals in one sale will be more appealing. Most don't really care what happens to the animal after they've pocketed the cash.
If you've got the stomach for a fight, go back to the farm armed with the evidence. If you still get the brush off, ask for their complaints procedure. If it's a charity, they will have a board of trustees, so you can appeal to them either through the complaints procedure (if they give you one) or directly. You can find out who the Chair is and the address from their annual report or probably elsewhere by googling. There's also the option of going to the local papers - play the violins about how your DC was upset that the farmer kept rabbits and guineas together when even DC knew that was dangerous for the animals. Tell them of a friend's experience (any of us on MN! :) ) of a guinea having to be put to sleep because of rabbit injuries, something like that.
The farm aren't going to want the bad publicity. They won't want the public to think they treat their animals cruelly, which in my view, is what they are doing. Good luck!