I came across this family fitness group meeting in Milliken Park. It requires no registration, is free, and open to all ages (they adapt the exercise accordingly). They sent me pictures and it seems like a really fun family outing. They have been offering it for three years, from May to October, twice a week. They put on upbeat music and provide yoga mats and hand weights. Funny story: there were no kids in the first set of photos the organizer sent to me. I asked her: "Where are the kids?" She said they had all gone to play in the nearby playground. Hey! It's still exercise, right?
Note that the meeting point is in the parking lot near the washroom building, off Steeles.