Suggest to them that you borrow the game from a friend or rent it from a store; once you have, they can look on as you play the beginning of the game. If they're up for the idea, they should see that it isn't a huge deal; if they're not, do as was suggested and simply don't get the game. Attempting to get it behind their back is only going to convince them further that the game is something that they would never in a million years approve of, and lead to unnecessary punishment for you. And believe me, they will find out. Parents always do.