The Erehwyna Destiny

It always bothered me in books like The Lion, the Witch, and the Wardrobe when the kids would have an adventure that fulfilled some preset destiny. Afterall, if they're destined to do it anyway, where does the heroism come in? Is it merely a matter of embracing destiny in a vaguely Kierkegaardian Either/Or sort of way, or is there a little free will mixed in with the determinism, and it isn't what the kids do that matters, but rather the manner in which they bring about their preset destiny that counts?