It doesn't "inevitably" happen for us. We homeschool and as far as I know our family and most of our friends don't make a lot of the Santa story, so I'm somewhat limited in the advice I have to give to those for whom it comes up often.
Having said that, I imagine that our kids would generally smile like someone who knows a secret. If asked what Santa was bringing them, they'd probably say, "I don't know what I'm getting" or simply "I don't know."
And before Howlin accuses my children of being liars again (and me of being a liar-maker), the fact is that Santa has personally been to our house before. Not the one with the flying sleigh and reindeer, but a damn good facsimile, in red suit and all. When he and his helper came to our door with gifts a couple of Christmases ago (sent by we still don't know precisely who!), they did in fact just smile and laugh.
Yes, they all knew he didn't come here from the north pole. They simply accepted him as the representation of the spirit of giving that he was. And they had fun, and enjoyed the experience, and appreciated the presents, the giving, and the spirit behind it no less for knowing that he probably drove, not flew, from some place more like North Powell Street than like the North Pole.
Actually, that isn't a bad response. But I am skeptical of the ability of a 4-5 year old to keep the secret from their friends. Even at 7, my son is having a terrible time keeping his presents to us secret.
The point was that those who don't do Santa with their kids (mostly) do not because they, in their words, choose not to LIE. Meaning, of course, that we must be terrible people because we LIE to our children.
Your choice is to teach your children to say nothing about Santa, be evasive about Santa or tell the truth straight out and make people mad. I don't view what I teach my children about Santa as a lie. Therefore, I wouldn't view your child who "plays along" with my Santa thing as a liar.
But I guess some people would.