I don't remember seeing pigs or cows at any zoos I've been too. Have you? I've certainly driven past cows in fields, but a child living in a city may not experience that. I think pigs are even more rare since they don't seem to be kept outside.
There's really no reason other than custom that appropriate numbers of pigs couldn't be raised and slaughtered in any city. They can subsist entirely on table scraps, the disposal of which is a regular burden for cities. They don't require lots of room to roam. They do produce effluent, but that is only a burden at mega-farm volumes. An individual hog will produce roughly what an individual human will produce, but only when it approaches the right size to butcher. Eventually this will seem normal, if only because chicken husbandry is growing more common all the time in many cities.
