I'm surprised that no one has mentioned Rangoli Indian Cuisine and Sweets. They are located in Orleans on St. Joseph Blvd. just east of Jeanne d'Arc. The naan here is the best in the city, and the food is without question the best Indian I have tasted in Ottawa. My second favourite is Coconut Lagoon (St. Laurent just north of McArthur) for the lunch buffet.
The Wilfrid Laurier hotel referred to in the first reply is actually the Fairmont Chateau Laurier. There is information on their website about Christmas dinner. I believe that there are two types: a buffet and a sit-down dinner. The hotel is located in the centre of Ottawa.
You might also want to consider the Brookstreet Hotel if your relatives are located in Kanata. Check their website for menus.
Winter weather in Ottawa is unpredictable, so take that into consideration when you are booking. Enjoy!