What I would say would depend on what I was disfellowshipped for. If it was for immorality, then I would probably simply say:
"I may be a sinner, Mom and Dad, but I'm not wicked. I love you both, and that won't change. Jesus preached a message of love and forgiveness, so take that into consideration if you choose to follow the message from someone other than him. I know for a fact that there are lots of very faithful Jehovah's Witness parents that do not shun their flesh and blood because in their heart they know Jehovah would never demand that from them. You better think long and hard before you allow a religion to put that kind of wedge between you and your flesh and blood. Give it some thought....that's what Jehovah gave you a brain for. I love you both. You may think I've left Jehovah, but I have not. I've left a man-made end times Adventist religion offshoot....not God. You better think about it for your own spiritual well-being."
If the reason was for apostasy:
"I can't open the door for you, Mom and Dad. The information is out there for you to examine just like the Bible tells us to do. Of course, the WTB&TS doesn't want you do examine it because THEY KNOW. If your faith in your religion is strong and you firmly believe it's THE TRUTH, then what do you have to lose by doing what the scriptures tell you to do: make sure of all things.
I love and appreciate your sincere efforts in raising me. My leaving this religion does not negate that and it does not mean I am abandoning you or ending my love for you.
This is serious spiritual stuff and all I can tell you both is "you better examine the facts before it's too late. You may find that the admonition to "get out of her my people" goes beyond just what you've been told by a few men writing doctrine in a high tower up in New York."