Actually, since we started going to Disney, Wonderland doesn't seem to be all that wonderful. Maybe it's because there aren't any fastpasses, and helpful signs that tell you that the line from here is 2 hours from the ride are common. Or maybe it's just that there's no real theming, no story to the rides. On the other hand, that may be the small part of me that is an adult that appreciates that more than the ride itself.
The rides themselves are good. There are lots of rollercoasters, including a stand up rollercoaster, 2 suspended car rollercoasters, and a backwards/forwards one. There are also some other thrill rides, as well as several conventional midway rides. So if they are heavily into rollercoasters, this might be a good (if a tad pricey for non-Disney) option.
Also, or perhaps as an alternative, if you're going to be in town mid-to-late August, is the Canadian National Exhibition (CNE). It has a midway, including some rollercoasters (not major permanent theme-park type, but still good), as well as other attractions, some of which are kind of Epcot-ish (international marketplace, different showcase country each year, new technology demonstrations), as well as animal shows. Includes admission to Ontario Place (notable for waterpark attractions and pedal boats, as well as an IMAX theatre).
http://www.theex.com/
And I suspect your kids will really enjoy Niagara Falls, which, in a way is one big theme park.
I hope you have a great time.
Ciao,
Mort.