How can I stop my timid senior cat from hiding in the litter-box?

Is there some way for her to have some sort of private area to herself? Maybe you could keep her closed in a room with her own box, bed, toys, water, food, and window for so many hours a day, or while your at work, or whatever, to where she has a break from the stress of the youngster. Maybe if they have more gradual exposure and she feels she has some ability to get away, she'll eventually adjust. Make sure you come in and visit. :)