1. There are approximately 2 billion children in the world. Since Santa does not visit Muslim, Hindu, Jewish, or Buddhist children, this reduces the workload to 15 per cent of the total, or 378 million. At an average (census) rate of 3.5 children per household, that comes to 108 million homes.
2. Thanks to the different time zones and the rotation of the earth, Santa has about 31 hours to work with. This works out to 967.7 visits per second and a total trip of 75.5 million miles. So Santa's sleigh has to move at 650 miles per second.
3. Assuming that each child gets only a medium-sized Lego set (2lb), the sleigh is carrying more than 500,000 tons. A reindeer can pull no more than 300lb. Even if the "flying" reindeer could pull ten times that, Santa would need 360,000 of them. This increases the payload another 54,000 tons.
4. 600,000 tons travelling at 650 miles per second creates enormous air resistance. The lead pair of reindeer would absorb 14.3 quintillion joules of energy per second each; they would burst into flames almost instantaneously, creating deafening sonic booms in their wake. The entire reindeer team would be vaporized within 4.26 thousands of a second, or about the time Santa reached the fifth house on his trip. Not that it matters, however, since Santa would be subjected to centrifugal forces of 17,500 Gs. A 250lb Santa (which seems ludicrously slim) would be pinned to the back of the sleigh by 4,315,015lb of force, instantly crushing his bones and organs.
5. Therefore if Santa did exist, he's dead now.