It's a huge assumption to say that all these parents are "lying" to their kids. I'll never agree with the anti-clause zealots in this thread.
They're letting the kids believe the whole Santa Clause concept. When is it appropriate for someone to be told he doesn't exist? Should I as a soon to be 26-year old next week continue to believe in him? That would make me idolizing another concept of God, would it not? A 5-year-old does not have the same development as a 26-year-old, but if no one tells that 5-year-old the truth the next 21 years, then sure enough he or she will be 26 still believing the same things.