Should I teach my young cousin how to break someone's nose?
You're joking, right? That seems to be too violent. Why would you want to teach him something like that? There are better stuff to teach in my opinion.
I want to teach him how to defend himself in real life.
You want to teach a child to defend himself violently, that seems to be something that could get him in trouble one day. Breaking a nose is illegal, teach him something else like martial arts.
Nah, I'll just teach him how to do it.
Rather than teaching him to do harm to others, like punching, why not look into judo or other forms of defensive martial arts? It's safer and also means your cousin won't have to hurt anyone. It's also cooler to take people down in groups with judo.