Well, I went and read the whole article and I still don't quite follow what excatly they're suing for - $100,000 each for mother and father and $25,000 for their daughter, but for what?
In any case, their claim that the city is somehow at fault for having trees on this hill is ridiculous - if anything it is obviously the responsibility of those going sledding, or their parents in this case, to choose a venue for sledding without any solid ojbects in the way that they might run into. AND, the city had signs posted that the hill was not meant for tobogganing.
When I lived in the Ottawa area years ago, I always felt that sledding was a dangerous sport, having broken both my arms on two different occasions. For that reason, I stuck to the sport we called "bunking," which required slick streets, a pair of ice skates, and a tolerant motorist, to whose car we would hang on as we were dragged all over the neighborhood.
None of those pesky trees to get in the way.