Cartoons aren't just for Saturday mornings. Now they're also about stepping out on Saturday night. NYC's 1 on g boutique sells Astro Boy tees, while Heatherette's Hello Kitty Couture clothes and accessories have found fans in Christina Aguilera and the Hilton sisters. Helmut Lang featured Felix the Cat tops and a dress on the runway, and Jean-Charles de Castelbajac just slipped a Mickey (Mouse, that is) on summer frocks. Pink-haired cyberpixie Eyeko appears on Nina and Max Leykind's eyebeauty.com products -- popular with Kylie Minogue and Cameron Diaz. The Peanuts gang appears on day-of-the-week undies (doeworld.com). Cartoons ''look back to an age of innocence,'' says Nina Leykind, ''and immediately conjure up a sense of nostalgia that people find comforting in uncertain times.'' Talk about staying in 'toon.