My children were 9 and 7 I think. It was easier because we left together so it could be presented as a family decision. With them we told them that we were having doubts about some of the things we had been taught and had passed on to them. We told them that it was easy to make mistakes but what was important was acknowledging when we made mistakes and try to make things better.
We told them that whilst we looked into things we would restore the family to the default setting (my son loves computers so got the analogy) and we would allow them to participate fully in all school activities and attend clubs that they thought they might enjoy. If we decided after research that we might need to make adjustments we would discuss it with them. The school was told what was happening and was advised that the children could fully participate the only criteria was that is the children felt uncomfortable participating in an activitiy that was previously prohibited then they shouldn't be forced.
The children adjusted very quickly to the new way of life. We have always encouraged them to ask questions and taken any of their concerns seriously. They have met many ex JWs and have seen for themselves shunning and anguish caused by bad religion. This has been more powerful than any lecturing. Neither have any wish to go back to the faith and have nothing positive to say about the meetings they attended.
Initially my daughter had concerns we would die at Armageddon but I asked her why she was concerned and got her to reason on where her belief came from and whether there was any evidence that anyone would be destroyed. What we found important is to allow time for them to ask questions.