Before dunking the doll in water, I would make a solution of water and mild soap and go over the doll's body with a soft toothbrush. That way you wouldn't soak the stuffing. If there is embroidery for the face, be very careful as the floss could run since it was manufactured a long time ago, although I've never had any runs with DMC floss (used to cross-stitch). Good luck, that is an heirloom.
Sue