You don't. Eventually they figure it out themselves (generally, one of their peers tells them the truth). They usually play along with the gag for about a year or so after they find out, then one day they tell
you.
Around the time they start noticing that there's Santas everywhere or they start trying to figure out the logistics of how he actually gets down the chimney, gives gifts worldwide in one night, etc., they're putting two and two together. When they start hunting for the hidden wrapped present stash the jig is pretty much up