“Fanboy and Chum Chum,” which premieres tonight on Nickelodeon before taking up its regular post Saturday morning, is a cartoon about two kids who live in a permanent state of playing. They dress as superheroes, wearing their underwear on the outside for that Superman look. (They have no out-of-costume alter egos.) Their collective mental landscape is littered with the detritus of sci-fi and fantasy, with the stuff of comics and movies, toys and TV shows. But we don't see the world as they imagine it, Walter Mitty-style; we just see them in their world, imagining.