I quickly took on the lead runner role dashing down the sidewalk with the five and six year olds. I let them know our stopping destination was the red fire hydrant at the end of the road so they had a sense of where we were going and it was in sight -- a good destination.
My three year old wailed because I was so far ahead of her, so some carrying and consoling was needed for the return run and another mom quickly jumped in as the lead runner. We had a 2 year old, 3 year old, 4 and 5 year olds and even an infant in a front carrier along on our run/walk.
We covered about a 1/2 mile and we didn't call it a race. The kids went out for a run just like their moms did. Active parents are great role models for kids and acting on a kid's interest in being active, too, is powerful stuff.
I just found a website about kids' running that is hosted by Runner's World at http://www.kidsrunning.com/. I'm going to check it out for ideas.