Off topic slightly
Parents tell this lie to there kids every year for as long as the kids believe it yet If the child tells the parent lies they are disappointed in their children and tell them its wrong. What makes this an ok thing to do?

Because parents aren't children ([daily mail]usually[/daily mail]?
Adults have a responsibility to face the reality of the world and children do not. Also, children do not have sufficient understanding of the world to form fully informed opinions. Children are free to go wild in their imaginations, as long as the "lie" isn't damaging their ability to grow into functioning adults, why not?
This is like the question of whether we should use children's opinions on elections or global warming or whatever. No, no we should not.