If the child is actually under arrest, I believe they have to cuff the child. I believe that is standard policy even for non-violent people and children placed under arrest. However, my understanding (although I am not a criminal lawyer) is that the police are only supposed to handcuff people who are not under arrest in order to protect the officer from possible harm from the person. I don't see any sort of circumstances where a 6 year old could pose such a serious threat to a grown man that the child had to be cuffed. That seems quite excessive to me if the child was not actually under arrest.
