I'm thinking of charging interest on the money my parents borrowed
Are you trying to teach your parents a lesson through this?
I'm clever this is why I know doing this will make me successful.
It may be clever and a business tactic, but they are your parents and you shouldn't abuse them like this. They took care of you for so long and bought you so many things you have the nerve to ask for more and more money. You need to be kinder to your parents.
Okay, when you put it that way I can see it is wrong.
I am happy you see that your parents are worth it. Family should be there to help and support each other in times of duress.