Stretching kits are terrible for your ears. They are made of inferior material that will cause tons of issues in the stretching process.
It sounds like you haven't done much research on how to stretch your ears properly.
Once you do more research on how to safely stretch your ears, tell her the facts about stretching piercings, and then ask if you can do it when you turn 18 because you will be a legal adult then.
You have already messed up by lying, and by purchasing a terrible product for it. Learn more about it, then talk with your parents.
If they say no, accept it and move on. The more you bother them with it, the more they will tell you no out of spite and nothing else.