Depending on where your nephew and niece are from, here are some suggestions they may enjoy: children's theater (e.g., there may be some shows at the Mississauga Living Arts Center), cultural activities (e.g., visiting Little Italy, Greektown, Chinatown, etc.), the Metro Toronto Zoo (still OK to be outdoors and I think they have some things going on around Halloween), Canada's Wonderland, Sega City, Dave & Buster's, Joe Rockheads, High Park, Fort York, and Casa Loma. If they are from really far away, I would say a trip to Niagara Falls can be a lot of fun too -- the Falls (of course), hiking or biking by the river, the wax museum, Rainforest Cafe (also at Yorkdale Mall), the artificial skydiving place (sorry, I don't know what this is called), Great Wolf Lodge, Fort George, a helicopter ride.