Actually, I'd say its that they haven't learned how to lie ineffectively. Crackpots are usually deemed to be cracked-pots when they no longer make enough sense to pay attention to. They argue by a self-standard of usefulness instead of adapting.

Well let me break down what I meant. The average adult who argues that Pluto is a planet honestly just wants Pluto to be a planet, but he knows that's not a good argument and knows to manufacture another, more convincing argument in its place. He even convinces himself that his rationalization is really the reason why he thinks Pluto should be considered a planet. He lies to himself, in other words.

Kids aren't necessarily clever or knowledgeable enough to think that far. But kids can and do lie, and they're not always effective about it.