Glad that some others have piped up and said that a supernaturally powerful child/young adult in a pivotal role in a campaign isn't childish or cliche. Why? Because I have just that in my campaign. The kid in question has just turned 15, and is currently under the protection of our PCs. He's not so much 'powerful', as he is touched by a god. He's destined to become the leader of his people and lead them in battles against an overwhelming foe (and to aid him in this, he has divinely inspired strategies and tactics). Of course, supernatural wisdom won't save him from a thug with a sword, so he's not that bad.
Some of the examples of the above remind me of Marvel Comics. First there was Franklin Richards, a la the Fantastic Four. The kid had enough mojo to scare off Galactus, who liked to munch on worlds. And then there was Nathan, the progeny of the founding X-Men couple, who was just as bad.