You won't have trouble getting the kids to show you how fast they can run - they love the opportunity to be speedy. Incorporate some change of pace type of things like the Indian running, a ball tag game that keeps them moving, but they can "rest" during parts of it, relays as mentioned, are great. Show them they can recover while still running. That is something that will benefit them whether they stick with running or move on to another sport. Anyone who can condition themselves correctly will be way ahead of their competitors at those young ages.
Any chance you could introduce some field events, too? Let them try their hand at long jump, throws and high jump if you can get the equipment and some expertise. I like the idea of running the bases, too.