Are we Teaching Our Kids its OK to lie?

One thing that happened to my six-year-old neice recently is that some of her first-grader friends told her that God doesn't exist. That we lied to them about other fictional creatures around Christmas and Easter and that we are lying to them about God too. We lied to them that as long as they are good... and we have done the same with what God says to do...

I have started to rethink my theology on raising children. One day soon, I hope to be a mother myself. Will I tell them these fictional characters are real and just hope they can tell the difference between lying to them and telling them about the one true God? I don't know...