After 25 years as a movie-studio executive, Joseph A. Adelman recognized a unique audience for a new theater chain. Kidpix Theaters Corp., in Los Angeles, caters exclusively to children ages 5 to 11. Its mall theaters, the first of which opened last November, show cartoons and other shorts to kids whose parents want to park them for awhile. A $3 ticket buys up to two hours of supervision. The theater is dimly lit during the show so the kids, who are supervised by college students, aren't spooked by the dark.