That's younger than I thought. It has been a while since I have actually seen a magic card. I have only played it once or twice and I believe it was the first edition of the cards, so probably limited but I don't remember any kind of sexual or overly graphic images on the cards. There are definitely some very graphic books in the D&D series that I have seen, though.
As far as using it as a tool for education I would support that. The strategy and forethought is excellent and will probably hold the attention of kids that age better than chess would.
My argument would be, this game is like chess with more depth. As far as demonic craziness, first of all if the parents in question are fundies there is way more graphic reading to be found in the bible than most fantasy genre games. But that issue is moot, they should trust their children with some degree of intellectual responsibility. If they're worrying about them playing Magic at 5 to 12, they have their work cut out for them at 13-18.