It helped put my sister into a really terrible Born-Again phase. She was even telling me about Satanic messages backward-masked in records. She got over it, but her husband's mother is an evangelical preacher (to a degree that disconcerts even other Born-Agains) and has inflicted Christian rock on their daughter. (That said, the husband is remarkably stoic and his mother has turned him into a passive-resistance agnostic.)
So, er, yeah: if you drop someone into an environment calculated to inculcate them with toxic memes, it might turn out to be as influential upon their thinking as it explicitly intends to be.
I am the son of a pastor, by the way. The issue may be what youth groups one goes to; not all of them are that virulently designed.
The best argument for Christianity is happy Christians and unhappy atheists; the best counterargument to that is not unhappier Christians but happier atheists. If you (and your children) already have what the youth group is selling, the danger should be seriously reduced.
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.