I think that if you want to do it and you enjoy it, then do it. Its your life. If you know its bad for you and are taking a risk and still want to do it, then thats your choice. Not your parents, not the government, not the doctors. Your choice. Just flat out tell them that.
You might want to look at google images for pictures of the effects of dipping tobacco and show your dad too. Pretty ugly viewing. No one can make you stop but you would be very stupid if you didn't try.
Yes, you should tell them, I would, cause they WILL find out somehow, may as well tell them by yourself so they have trust in you to tell the truth. But if they find out another way, first of all, they may be more angry at you and have less trust, second, depending on the parents, you might get in less trouble than them finding out in a different way.
I truly think you should at least TRY to quit, because it gets expensive and even harder to quit as you go, but I won't nag on you.