Wife and I were all set to head to Belgium in November then the terrorist attack happened. By random selection, we decided to head to Toronto instead and were pleasantly surprised by how much fun we had. (Note: we live in NYC, so the cold doesn't bother us.)
If your kid is into trains, there's an old train depot halfway between the CN Tower/Rogers Center (Blue Jays) and Steam Whistle Brewery. As the previous poster mentioned, definitely do the Steam Whistle tour. For $10(?), you get a 30 minute tour, 2 beers (minimum) and choice of beer mug or shot glass afterwards. Also, buy some Steam Whistle to take back to your room with you. Maybe your kid could wait outside with your wife?
Or, the 2 of them could go to Ripley's Aquarium (recommended only if you haven't been spoiled by the Georgia Aquarium) across the street.
Distillery District is solid for dinner/drinks. The Momofuku previously mentioned is also very good. I probably would have been comfortable taking either of my kids (2 and 5) to those places but, then again, we eat out a lot with them. Carmen's Steakhouse was also very good.
Day trips to Niagara Falls are exhausting but worth doing once in your life.
We saw a Leafs game when we were there and it was a great experience. Definitely visit the old Maple Leafs Garden- there's always a few hockey games going on there regardless of the season. No cost to get in.
Try to take the subway everywhere you can. It's efficient and clean.
Not sure how the USD is doing right now, but you can do a lot of good shopping if the dollar is still strong.