My daughters have been to my 2 races, and are becoming interested in fitness. The older ones are 9 and 7. They like to run with me, so we do about 1K together, then I do my run or bike. The run with the kids is a great warm-up.
On Saturday, we did a little aquathlon. A swim to the raft and back, and a 500m run, with a transition area on the patio. What a great time.
I'm hoping the girls get in the habit of exercising, even at this age. Heck, I was 10 when I started competitive swimming.