That is so great. I share the feeling. Last summer my girls 7 and 9 did a kids triathalon. The distances were quite short but they had a blast. The key is to make sure it is definitely easy enough for a kid to finish so they are happy when they're done, not exhausted or god forbid in pain.
My 9 year old is doing "girls on the run" so she has now run 3 5K's. She is super proud of herself. I'm proud of her too!