My dad was a banker and he always spanked me from the day I turned 7 until I was 10 whenever I spoke or acted out of line; at the time I resented him greatly for that but now I see how it made me a better person, it taught me some respect and obedience, so that isn't just limited to police.
If it's wrong for a regular person to do, it's wrong for a cop to do. Physically disciplining children is not an effective teaching technique - it's immoral. As long as our society maintains the tradition of using violence to attempt to modify behavior we will continue to see violence as a result.
Never mind children, police should not be allowed to "physically discipline" anyone. It is their place to protect the people and enforce the laws, however, it is not their place to actually punish crime. Police must do only what is required to stop crime and detain criminals. The actual discipline is the responsibility of the legal system.
It's not the job of police to punish children for their misdeeds. That's the job of parents. If a child is charged with a crime, then it becomes a judge or jury who punishes the child for breaking the law. Police should only intervene if lives are at stake. If a kid has a gun and is about to kill people, then unfortunately the police officer should intervene simply to save lives and nothing else.
No, police should not be allowed to physically discipline children. The training that police officers receive is primarily geared toward the adult offender. They are trained to intervene in adult situations. The training they receive and the behaviors they are subjected to on a daily basis require them to respond on a level far more aggressive than what should be used on children. Police officers should not be placed in roles requiring them to perform at levels they are not specifically trained for.
I don't believe police officers should be permitted to physically discipline children because it would cause a number of legal problems, along with ethical dilemmas. Parents, if anybody, should be the only people allowed to physically discipline their children, so long as it is within the bounds of reason and the law.