Kill your parents..
on a serious note, explain to them why you want to build it and try getting them to see your side of the argument. Skiing is a sport that means something to you and you want to practice and progress in the summer, even if the local hill is closed. I feel like a lot of parents dont think that skiing on handrails really classifies it as a sport and to them you making a backyard setup just seems like an easy way to get hurt. Try to tell them things that will give them peace of mind ie) wearing a helmet, putting a mattress down on the landing etc. explain the benefits and you should be golden. Cheers!