Ever heard of READING to your kids? You can start that in the cradle - I was reading to my daughter in the womb, for heaven's sake.
My parents got me the full set of the Narnia books at age FOUR. It was amazing how fast I learned to read so I could find out What Happened Next without waiting for Mom to read the next chapter.
I don't recall anybody drinking a potion in The Lion, the Witch, and the Wardrobe. There IS a witch of course, but she is obviously and unrelievedly evil - she has enchanted Narnia so that it's "always winter and never Christmas" . . . The witch does give Edmund enchanted Turkish Delight that appears when she pours a potion on the snow, but (of course) it turns out to be poison. Lucy is given a cordial that heals wounds, but I don't recall anything else. What's with the Sun worship? I don't remember any of that.
If you really feel that none of the trappings of romance can be "Christian", you better not read Paradise Lost, The Faerie Queene, or any of the medieval legends. Of course that is the tradition Lewis draws on (in his day job he was a professor of medieval and Renaissance literature at Oxford and later at Cambridge) and it is absolutely, undoubtedly, and thoroughly Christian -- much more so than the modern age, believe me.
Well actually I'm reading and teaching my grandkids. We have them for the summer. See their parents are both TDY in the Air Force. But I can't get them to read Narnia quite yet.
Me, too - and that's the only time I've been able to read to any of them without constant interruption!