My nephew is three, so right now it's not really anything I tell him. I try to encourage things that will, hopefully, in the future, promote learning and thinking. Right now he is big into dinosaurs and bugs. I buy him all kinds of dinosaur toys and *books* with *facts* that will, maybe, one day make him question silly doctrine that doesn't fit with it. I also just bought him a really cool ant farm that he loves to watch with gel that they burrow through so you can see all the tunnels and such.
When he's older it become harder. What do I say when he's 8 or 9 and he asks why I don't believe in Jehovah, or go to the meetings, or why I celebrate the holidays, or whatever it is he asks? He is still his parents son at that age and I wouldn't want his parents to feel like I was trying to pry him away from their beliefs because then I lose contact with him and if he ever wants out later in his life he has no lifeline. But I also don't want to hide or lie. It's going to be a difficult tightrope to walk.
I think a lot depends upon the age, the maturity, the relationship, the timing, how rigid his parents are... there are just so many factors. I don't think there's any one specific answer.
Jackie