1) No known species of reindeer can fly. But there are 300,000 species of living organisms yet to be classified, and while most of these are insects and germs, this does not completely rule out flying reindeer, which only Santa has ever seen.
2) There are 2 billion children in the world (persons under 18). But since Santa doesn't (appear to) handle Muslim, Hindu, Jewish, or Buddhist children, that reduces the workload by 85% of the total-- leaving 378 million according to the Population Reference Bureau. At an average (census) rate of 3.5 children per household, that's 91.8 million homes. One presumes there is at least one good child per house.