Erithtotl said:
Some good stuff here, and some funny comments too, I like it.
The staying idea is actually really cool. I would consider it but in our campaign the party is actually serving a higher power, and their main mission can't really be put off.
You have your answer, from my perspective. Talk to your DM about this, as your paladin would know the rules of his god well enough to determine the best (not the "correct") course of action. My logic is as follows:
You are engaged in a higher duty to your god currently, one that the world cannot ignore. What good is it to save these 40 children, if, by your lack of participation, the whole world is forfeit? On the other hand, the children have NO ONE, not even the slavers, to care for them.
The best alternative is that you send a message to your order, through the PC's, informing your superiors of the situation. You stay behind to see to the children's well-being, and make preparations to either bring them to the mainland, or to set them up with more permanent instruction here. (I am opting for the "bring them with you" aspect myself, but your sea captain is going to balk at bringing FORTY extra passengers with you without having planned for them.)
Your PC buddies go back to the church with you, and another paladin or holy warrior is issued to your group as a replacement. VOILA! You play the NPC for the balance of the "crisis" quest, then after the quest is over, you resume play of the old PC. If your DM is feeling generous, (s)he could just say that the experience you accumulate playing the NPC gets transferred to the PC, representing the experiences that your paladin had in the land of savages protecting and converting the children.
This way, your paladin follows his heart without harming his duty, your experience totals don't get too far removed from the other PC's, and you don't have to sit out any action.