Wow, i did this exact debate a couple of weeks ago! except it was that 'parents should NOT be able to smack their children' and I was affirmative

haha
Okay, well:
Im not sure what the reference to NZ is but I have loads of information from the Affirmatives side for you if you want
I'll copy and paste now
these are things to rebut:
A corrective smack on the bottom from a responsible and loving parent is clearly different from a fist in the face or blow on the chest or back. Anything less than a ban on smacking means many parents will continue to assume that hitting their children is their only option. In fact there are many other ways of disciplining children which are more effective and which teach children self-discipline without hurting them.
Parents should be giving children the right messages about the use of violence. No one wants a child to grow up in a violent society but, inside the very place where they should be safest, parents teach them that hurting someone is the answer to problems.
Parents should be teaching children the difference between right and wrong and helping to reduce the level of violence in society rather than contributing to it.
Smacking children does none of these things.
A number of prosecutions for parental smacking or restraint have been reported over the past year. These have caused great distress to the families involved, especially the children. They have also wasted the time and resources of courts and social services, as well as public money, all of which is urgently needed to help seriously abused children.
Current laws let parents hit children so long as it does not leave a lasting mark. This is wrong because it allows blows to the head and means kids who do not bruise easily can be hit harder. The law should be that nobody has the right to hit children.
Children are protected by law from harsh punishment and violent abuse by parents and others. It is the duty of society as a whole to protect its most vulnerable members from harm. If an adult hits another adult it is considered assault, a crime, and therefore becomes a legal offence of the law. Why then should a full grown adult hitting a small child be considered a 'domestic activity' and not be punishable?
A number of prosecutions for parental smacking or restraint have been reported over the past year. These have caused great distress to the families involved, especially the children. They have also wasted the time and resources of courts and social services, as well as public money, all of which is urgently needed to help seriously abused children.
Current laws let parents hit children so long as it does not leave a lasting mark. This is wrong because it allows blows to the head and means kids who do not bruise easily can be hit harder. The law should be that nobody has the right to hit children.
Children are protected by law from harsh punishment and violent abuse by parents and others. It is the duty of society as a whole to protect its most vulnerable members from harm. If an adult hits another adult it is considered assault, a crime, and therefore becomes a legal offence of the law. Why then should a full grown adult hitting a small child be considered a 'domestic activity' and not be punishable?
If you want anymore then I have some but yeah

Thats some of my points on why it's a criminal offence