The unknown is a scary thing. Which is why everyone harbors some degree of fear for it. Right now, your parents are a known quantity. You can safely put them in a certain category, and they you.
People react different ways when they are met with an event that suddenly pulls someone out of one category and into another. Are your parents people who have had someone in their life suddenly do something that they disapproved of and changed their behavior towards them? Perhaps this is what is giving you some pause.
Disapproval is something that people also fear. We are social creatures. Even hermits adopt some form of socialization with their environs. We want to be connected, loved.
If your parents will react badly, perhaps you shouldn't tell them at all. Live your life how you desire. If they happen to walk in on you doing something, then confront the issue. Don't hide, don't make it seem like you're ashamed. If you do this, they most likely won't make a big deal of it.
Should you tell them, keep it simple. Don't throw blocks of information at them because you're nervous. If they ever walk in and question something you're doing, say, "Oh, I've been preparing a ritual for x-thing to help with y-thing." This leads to a specific sort of questions where you don't have to decide what they're thinking.
Good luck. Live your life towards your own happiness. You can't spend it trying to make something else happy at the cost of your own. You'll either begin to resent them or yourself, leading to the loss of both parties happiness.