Wen I take my son to the park, I push him on the swing. He likes to swing high so that's even more of a workout. While pushing I do lunges, squats, calf raises, and various other exercise. I will also play tag with him on some of the larger playgrounds. Get outside and be a kid! They love it!
Since the weather is nice, I take my three year old to the park often. While pushing him on the swing I do some kind of exercise, lunges, hip flexors, squats, calf raises and things like that, while, at the same time I am getting arm exercise by pushing. Great mini workout.