How Do I Respond to a Confused Four-Year-Old?

Questions about what to say to a confused four-year-old grandson whose father is telling him there are hundreds of gods, why it’s difficult to persuade a Mormon or Jehovah’s Witness, and how to engage someone who claims to be Christian yet attends a Unitarian Universalist church.  

• How do I respond to a confused four-year-old grandson whose father is telling him there are hundreds of gods?
• Why is it so hard, when having a discussion with a Mormon or Jehovah’s Witness, to help them understand and set them free with the truth of the gospel?
• How do I engage someone who claims to be a Christian yet attends a Unitarian Universalist church?