You asked the right question in that it's HOW you tell them, not what. You have two motivational forces right, fear and love. Take care that you don't just act out of fear, because even if it is a fear that you are under mind control that is still fear. The whole question of what to say is just a reflection of the JW mindset, talk is cheap. I'm sure you'd agree that as others have pointed out, what children need from their parents is love. Don't be too concerned with cutting through to the truth, because if anything kids need that support more than anyone, and also when all is said and done love is the point of it all isn't it? Kids atleast aren't as corrupted as adults when it comes to this, they'll be able to tell that it's genuine. Just don't get too caught up in the information, because that's all its good for. It has a place of course, but you're concerned about your relationship with your child - information you can get from a book, but we don't have relationships with facts.