I believe Atheists that expose their children to faith systems do so in a more or less anthropological way, as in "look, there's other people in the world with other beliefs, and you should draw your own conclusions and respect them."
While many Christians might agree with the idea of respecting others, you must remember that they believe in souls, and that a choice such as atheism would lead to an eternity of pain for their child. Therefore, Christians (or members of other religious sects) do not encourage their children to "decide for themselves"... they try hard to indoctrinate the decision into them. In their eyes, they are protecting their children. If they introduce atheism as a topic at all, it will be in a derogatory fashion in order to indoctrinate their child(ren) into loathing atheism. Luckily, some kids still manage to think for themselves in the end.