I don't remember any trauma, or honestly even transition from "believing" to understanding the story my parents did use to explain.
Like some of the PPs have done, my parents used Santa as a representative of giving and thinking of others. I don't remember exactly how they phrased it / told us, but the gist of it was that Once Upon A Time, the world was much smaller. There was a Santa/St Nicholas who made sure that everyone in his town had a Merry Christmas, with presents and good deeds. Then the world got bigger, and Santa could not do it by himself any more. So, as soon as we are old enough, it is everyone's job to be Santa for all of the people they know. [This didn't translate into *presents* for everyone we know, but kindness and good deeds for everyone.] And, as long as we all treat everyone this way, there is always a Santa Claus. And if we're very good, then Santa is here all year round.
I like it, and I'm sticking with it. :-)