It would be easier to judge that if we knew what time they were currently showing South Park. If they are showing it at 7:30pm I can certainly see the problem. If it is at 11pm, when the children should be in bed anyway, that may be a different matter.
For that matter, if there are only a couple of free to air channels so little alternative for parents who don’t want their children to watch it that could be a problem, but if there are 20 free to air channels with other options for children to watch at the same time there would be much less of a problem.
We don’t have the full story, there may be more in behind the request for a ban than we realise when we consider it from a NZ perspective.