I believe you need to take a different approach.
Try to show the kids Bible verses that the jw's interpret wrong. The jw's have a few doctrines according to the Bible and some doctrines that are totally against the Bible.
A person who studies the Bible will never believe you if you say "God does not exist" but will think twice if you say "look what the Bible says here".
Even if you do not believe in God anymore, do some research on this to help your kids.
here are some resourses:
http://www.biblicaltruthseekers.co.uk/
the site above even has a section dedicated to jw's - and where they are wrong
Another resourse:
Contact me if you need help.