I hate repetitive exercise. My solution is to use walking, hiking and more recently using a Trikke because I’m going somewhere and attending to the changing environment around me, so I don’t get bored. Also, when using gym exercises Ill get bored or tired and stop. If I’m out on my Trikke or a hike, well… tough luck if I’m bored or tired! Sitting down and giving up isn’t an option, because I still have to get home! I suspect the same principles would apply to kids.