What if Carrie, Samantha, Charlotte and Miranda popped out a few kids? You'd have a hit or so HBO hopes. Sources say the cabler has quietly begun production on an as-yet-untitled half-hour comedy pilot that is being billed as Sex and the City for the Mommy and Me set. "It's about being a new mother in New York City," confides our mole. Working title: "Sexless and Bound for the Suburbs."