I faded when my son was 7. He was very happy to be able to have a real birthday party. I recommend talking frankly about your issues with the "former religion". There are too many scary stories out there about relatives bringing the children back in. I worked hard to innoculate my son against all that cult Bullshi&. You dont have to use the word cult- but explaining why certain doctrines made no sense to you any more helps alot.
I thinks that children are smarter than we give them credit for- and do not (contrary to witness belief) naturally believe in God. Children are naturally inquisitive, reasoning, and questioning. Only when their parents present something to them as "the truth" (repeatedly) do they accept it. Presented with openness and questioning they do much better.