It's true that the Mario games, such an icon of childhood, had some darker undertones (the whole mushroom obsession, for instance). This is not new. Anyone who has read a Roald Dahl novel knows that the finest of children's fare is often dark. Even Disney can be added to the mix, with movies such as Fantasia and Beauty and the Beast exploring serious themes. Children are more than capable of understanding subtlety, if not in the same way as adults, which is why these sorts of darker entertainments have a richness that is appealing to them.