''I've always gravitated toward stories with parents,'' says Steve Martin, whose last stab at hapless dadhood was the spring smash ''Bringing Down the House'' with Queen Latifah. ''Parents are emotionally complicated and very relatable -- as opposed to loser cops who are divorced,'' laughs Martin. ''Anyway, I like playing them.'' This winter, he plays pop to -- gulp -- 12 kids with Bonnie Hunt in the Fox remake of 1950's ''Cheaper by the Dozen.'' Here, Martin is trampolined into the air when a backyard Moonbounce bursts. What emotionally complicated parent couldn't relate to that?