The thing is, though -- the paladin operates by very strict moral codes, like Kant's ethics. In the moral question "Kill a child and save the world, or save the child and the world dies", the paladin may *NEVER* choose to kill the child, according to his moral codes.
Morals can change over time, depending on circumstances as well as how one would interpret their outcomes. Paladin's are the "chosen", yet in all other aspects human paladins are only human. Humans faulter, and make mistakes...even those who were chosen for having strict moral fortitude when it comes to their beliefs.
In this situation, the Paladin makes a decision...being a rationally thinking person with a strict moral code, and not just some "belief following machine". If there were no other way, the Paladin must choose between sacrificing the child, as well as his moral standing for something greater than himself, or let the child live, and the world end (which might just take the child along with it regardless). Yet he may also choose to let the child live, and the world end leaving fate in his god's hands. But if his god were slain prior to the decision, that would change things entirely...
Alignments shift with character's actions. Different characters follow and adhere to morals and beliefs differently than others (One paladin is not exactly as devote as another). There is also no set path one can take, regardless.
The paladin will face consequences for either action, whether being responsible for the destruction of the world, or going against his beliefs and killing the child in order to save everyone else who's fate rests on his shoulders. If he does the latter, he'll fall from grace...