Exposing children to different religions is extremely important, because hopefully what we are doing is illustrating that different people have different ideas, many of them opposing in ways, and so the child can examine that and learn to think critically. Of course, thinking critically is the greatest threat to religion, so it wouldn't be surprising if the child ends up agnostic or atheist!
Additionally, I think that learning about prominent religions is important because of the influence they have on society. I live in the U.S. and Christianity is (obviously) very influential on how people act, who they vote for, etc. It's important for someone growing up in this society to understand the beliefs and motivations of Christians, because they are going to encounter Christianity's influence so frequently. (I also live in Michigan, so some understanding of Islam probably wouldn't hurt, too!)
My stepson is 13, and his stepmother and father have taught him all about the Jesus mythology. (His father, by the way, has been arrested many, many times for things like assault and failure to pay child support. He has a record of violence against women specifically. Classy dude, and wonderful Christian!)
So, my wife and I ("evil" agnostics with no history of violence or arrests*) asked him one day if he believed it. He said that he did. So I asked him a very simple question: "Why?"
He was quiet and I watched the wheels of his mind turning. I think he finally said something like "I don't know."
So I said "A lot of people believe that story, and believe that Jesus was God. And maybe it's true. But it's also possible that it's a made-up story."
The only thing I've ever driven home is that I want him to research, think, and make his own decision when it comes to spiritual or religious belief. He could end up a Christian, Muslim, deist, atheist, whatever, and it will be okay with me, as long as he got there by thinking and making his own decision.
*In the interest of honesty, I guess I have been arrested once. For unpaid traffic tickets. Yeah, I'm a hard one.