In I, Claudia, her playwriting debut, Kristen Thomson wears a variety of masks to evoke a 12-year-old child traumatized by her parents' divorce. Her fractured world includes a recently widowed grandfather, a neurotic stepmom-to-be and a philosophical immigrant school caretaker. Don't expect the masks to hide Thomson's talent, though. In roles like the reckless youngest sister in The Memory Of Water, the manipulative student in Oleanna and the earthy Stella in A Streetcar Named Desire, she's burned up the stage with focused intensity. Still, her most memorable role remains a Dora-winning turn as Denise, the ferocious young mom intent on reclaiming her baby in George F. Walker's Problem Child. Let's see how she handles this new problem child. For details, see listings.