Before they've even figured out that beliefs can be wrong? I say parents teach them that beliefs can be wrong with Santa, and teach them that desperate desire to believe is not truth with God.
In a perfect world (and the way I would like to raise my kids) it would be the other way around, because God is more obviously wrong than Santa (Santa might break the rules of physics, but he gives you presents; God breaks the rules and does nothing), and Santa actually rewards belief where God doesn't. So I would introduce them to the God hypothesis, let them test prayer, and decide that beliefs can be wrong. Then try them out on Santa, see if they can find the truth despite being offered presents to believe a falsity.
But the important distinction, the critical distinction, is teaching them. When you teach them about motivated cognition, they've got to have the option of getting it right. If a parent withholds presents from a child because they refuse to believe in Santa, and Christmas comes and goes, and the child sees all their friends with presents and they have none themselves, next year they will lie for presents. Social pressure, parental pressure, just wins at that age. Kids usually start deconverting themselves later than single digits.
I've long entertained a dubious regard for the practice of lying to children about the existence of Santa Claus. Parents might claim that it serves to make children's lives more magical and exciting, but as a general rule, children are adequately equipped to create fantasies of their own without their parents' intervention. The two reasons I suspect rest at the bottom line are adherence to tradition, and finding it cute to see one's children believing ridiculous things.
Personally, I considered this to be a rather indecent way to treat one's own children, and have sometimes wondered whether a large proportion of conspiracy theorists owe their origins to the realization that practically all the adults in the country really are conspiring to deceive children for no tangible benefit. However, since I began frequenting this site, I've been exposed to the alternate viewpoint that this realization may be good for developing rationalists, because it provides children with the experience of discovering that they hold beliefs which are wrong and absurd, and that they must reject them.
So, how did the Santa deception affect you personally? How do you think your life might have been different without it? If your parents didn't do it to you, what are your impressions on the experience of not being lied to when most other children are?
Also, I promise to upvote anyone who links to an easy to register for community of conspiracy theorists where they would not be averse to being asked the same question.