I would show them all the isms. Both theism and atheism. I would try my best to allow them to decide from all the major belief/non-belief systems. Christianity, Buddhism, Islam, Hinduism, Judaism, Atheism, Sikhism, etc. I would make sure they know and avoid the nasty ones like the JWs, Scientology, the Cult of Satan, Satanism (yes, they really are two different things, one is a commie/bigot/business religion the other is the proper thing), any anti religion organisation (Anyone that is set to, wants to or wishes they could destroy the beliefs, faith and hope of the World are just plan **** heads.), suicide pacts (yeah, some of these I would save for a later age that is more ideal for education in the subject of the particular organization/belief system), neo Nazis, etc. I would want my children to know about everything, know the good, know the bad, and then decide what is right for them. And if they want to be agnostic then so be it. Whatever they would want I would support them through. I would want them to have a sense of justice, positive morality, strong common sense and to know what is right and what is wrong.
I would give them a choice. The freedom to choose without having to suffer one sided comments.