You don't. Your parents made a good call. 1) Knives are horrible, dangerous self defense weapons. Your chances of being killed or seriously injured while using a knive in self defense are exponentially higher. 2) If you have to say "How do I convince my parents" You are not old enough, or mature enough to be carrying a deadly weapon. Simple as that. Children do not understand the ramifications and potential repercussions of using deadly force. Your best defense is your legs, and your brain. You use your brain to identify a potentially volatile situation or environment. And you use your legs to take yourself away from the situation.