I have a five year old and he loves watching and mimicking my exercises. Even though kids are active usually in their own way, I find he loves observing and copying the structured routine of it. Ditto on the suggestion of pull-ups at a playground - or just a 'Grok hang'. As for the sprinting, go to an oval or playground and sprint around in circles - you don't have to go far away from your kids.
Primal since May 2012. Loving life and down 50lbs.