It is commonly recognized that we have a duty to be truthful and yet millions of Americans engage in the most elaborate deceit imaginable all aimed at duping their children... Santa Claus is portrayed as real, not only in the story but also by the parents. No parents pretend that Darth Vader is real but when I was on a plane on Christmas Eve the PILOT announced over the intercom that he had spotted Santa on the radar!!!! And, while it may be Ok to omit certain information in order to protect a child it is absolutely immoral to actively perpetuate a lie.
However, I don't really think this is (necessarily) lying. Such an interpretation would be excessively literal-minded. Not all statements are genuine assertions, meant to be taken literally. Kids are well versed in pretense, as Chris explains at Mixing Memory:
Cognitive psychologists, especially Jaqueline Woolley, have developed rather sophisticated ways of understanding children's ability to distinguish fantasy and reality. As I've discussed before, children are pretty good at separating fantasy from reality, but in cases of acceptable fantasies like Santa Claus and imaginary friends, children seem to exhibit a third ontological category, "pretend," which they have in addition to "real" and "unreal." While there are individual differences in children's ability to make the fantasy-reality distinction, overall, by about age 3, children are pretty damn good at telling pretend from real, even if they may play as though the pretend is as real as anything else.
So, I think it's a good thing for adults to engage children's imaginations by pretending with them that Santa is real. I agree that parents shouldn't deceive their kids, e.g. into thinking that Santa is real in the same way as Grandma. But children are sensitive to playfulness, and should pick up on the game if their parents play it right. At least, that strikes me as the ideal. To joylessly pop the pretense is not to advance the cause of truth and learning. It's merely to call a halt to play.