That has been an issue for us as well. There was a time around pre-school / kindergarten when my son was having a lot of conversations in which little kids told him (seemingly with the best of intentions) that he was going to hell, based on what they had heard from their parents. It took him a while to develop any kind of filter, or a concept of "choose your battles." Have you seen The Invention of Lying with Ricky Gervais and Jennifer Garner? Those people had no filters either, going well beyond deliberate falsehood in their definition of lying.