Are you fucking SERIOUS? Let me guess, you're one of those kids who threw a temper tantrum when you found out Santa didn't exist and accused your parents of being filthy liars. And now, because you're all bitter about Santa not existing you feel you must ruin it for those who really do believe; especially a young kid no less. :| What if someone who knew (hypothetically) the truth about the afterlife and it turns out there is no after life; would you really want them telling you that after putting in a lot of effort to be a good person? No? Then why should YOU wreck it for your nephew?
When I was a kid, I believed in Santa but I didn't throw a fit when I found out he wasn't real. Hell, I saw it as a nice little learning experiance about how good behavior does bring along karma.