I might be asked, ‘Do you equally reject the approach which begins with the question “What do modern children need?” — in other words, with the moral or didactic approach?’ I think the answer is Yes. Not because I don’t like stories to have a moral: certainly not because I think children dislike a moral. Rather because I feel sure that the question ‘What do modern children need?’ will not lead you to a good moral. If we ask that question we are assuming too superior an attitude. It would be better to ask ‘What moral do I need?’ for I think we can be sure that what does not concern us deeply will not deeply interest our readers, whatever their age. But it is better not to ask the question at all. Let the pictures tell you their own moral. For the moral inherent in them will rise from whatever spiritual roots you have succeeded in striking during the whole course of your life. But if they don’t show you any moral, don’t put one in. For the moral you put in is likely to be a platitude, or even a falsehood, skimmed from the surface of your consciousness. It is impertinent to offer the children that. For we have been told on high authority that in the moral sphere they are probably at least as wise as we. Anyone who can write a children’s story without a moral, had better do so: that is, if he is going to write children’s stories at all. The only moral that is of any value is that which arises inevitably from the whole cast of the author’s mind.
– C. S. Lewis, “On Three Ways of Writing for Children,” Of Other Worlds: Essays and Stories.
(I would also apply this advice to the work of Christian artists seeking to consciously impart a “Christian” message to their art as well. Rather than starting from the burden of “How can I make my art win people to Christ?” rather let the Word which pervades you overflow into your work as you create it. Also see this quote from Francis Schaeffer on Christians, art, and the totality of life.)