Back when the kids were 4 and 2, we flew down for Labor Day weekend. Being as young as they were, we were able to convince them that we were just taking a casual plane ride and then coming back to Atlanta. When we arrived in Orlando, the Disney Express bus wasn't labeled with anything Disney, so we convinced them that the bus was bringing us back to our car. We were golden up until the point they started playing a Disney World video on the bus. Had to tell them then, but the surprised look was priceless.
That's lie-layering. You build up a solid foundation of lies so that, when done, you can stand on top of it, look around, and say to yourself, with much satisfaction, "Yeah.. I think everyone is going to believe this. Good job, me!"
Kind of like when you tell your kids there's not going to be any Christmas presents this year, and they ask, "Why?," and you tell them, "You were good, but you weren't quite good enough..." Then when they look at you, not quite believing you, add, "Look, I've already sent the letter to Santa explaining the situation, signature required upon receipt. He signed for it. It's done."
At this point you've brought Santa into the picture. If you're smart, you'll have a copy of the signature receipt (just pick one up from the post office and pull the part that would be sent back to you out) already signed, "Santa." If this is an elaborate lie then Santa's in on it, too. They'll know this but still think Santa's a good person and that you, being their parent, are probably good, too. You've just provided yourself cover - Santa.
You'll have them crying for months! You can either pocket the Christmas money or, if you feel so inclined, pick them up the most mediocre present from Walmart and place it under the tree for Christmas morning. When the kids see it their eyes will fill with delight and you can say, "Looks like Santa threw you a bone. Merry Christmas!"
This is Parenting 102.