I've been there for work but didn't do any kid-friendly activities. Off the top of my head you may want to look into their ballpark, which I hear is good, and I think Niagra Falls is close. HTH!
I'd post this on the Canada Board. Really nice ladies there - it's not super busy so it might take several hours to get some responses, but I think you'll get some great answers.
DHs family is from there so we go once a year. Definitely go to Niagara Falls, it's only about an hour or so drive, depending on traffic. Center Island, they have rides and it's really pretty also. There's a few other things but I can't remember off the top if my head, since we spend so much time visiting family when we're there. Check out travel sites like Fodors and Frommers they always have good suggestions!
Re: Who has been to Toronto?
I spent a bit of time there before kids...
I'd post this on the Canada Board. Really nice ladies there - it's not super busy so it might take several hours to get some responses, but I think you'll get some great answers.
https://community.thenest.com/cs/ks/forums/3230708/ShowForum.aspx