Replying to Convincing Parents

So my parents won't let me backflip and they think I need a coach. Despite the fact that on a trampoline I can land a backflip every time, and my backflips on the trampoline are the ones that you jump staight up and tuck at the peak of your jump. Also they think I'm going to break my neck or something like that. So what do I do to convince them to let me do a backflip and that I don't need a coach, what did your parents think of you backflipping, how did your fist backflip go, and did you have a coach?