Santa Claus IS real. Even though we were adults and my parents were adults, my grandmother always did the santa claus thing. Her motto was "You don't believe, you don't receive." She even gave us easter baskets on easter. And with it we always got a new easter dress, an easter outfit and a new bathing suit to start the summer off with.
It was just more fun for us and for her if she got to write from Santa on all the gifts. Of course we knew that he wasn't real, and that she spent hours at a time in packed stores for 5 months before christmas to get all of our gifts, but where's the fun in that? Thats one thing I miss about her. Also, when I was 19 and had my wisdom teeth removed, my mom brought me a dollar bill and said "Here, .25 for each tooth, Toothfairys broke this week."
If they are going to tell her, then just sit her down and tell her. It doesn't mean however that they can't continue doing the Santa and Easter Bunny thing on those holidays. It can be a fun tradition for the whole family.