I have been coaching grade school track for a few years (Grades 1-8). We start off with a lap or two to warm up, stretch (dynamic stretching is good), plyos (which kids find fun - good work for sprinters), 2-3 laps of running the straights and jogging/walking curves (start off at about 60% speed, moving up to about 90% speed, stressing they work on form - knees up, good back kick, head still looking straight ahead, arms moving forward as opposed to across the body, which many kids do). We will also reverse that at some practices to running the curves and jogging/walking straights so they can practice "hugging" the curve. We then break into groups to practice field events, starts and distance training. We also practice hand-offs in relays at some practices. Is there a short, steep hill where you practice ? Powering up hills is good work for sprinters, and kids love running down hills just for fun (wheeeeee!).
The key is to try and get lots of assistant coaches to help keep the kids moving and focused when they are in their groups. See if you can't get some parents to help. Some parents are wary of volunteering, saying they know nothing about track. I try to recruit them as shepherds - just keep them in a group and moving. The more practices they attend, the more information they absorb, and then they find themselves able to coach particular events.
To finish the practice, we take a cool down lap then stretch with a few sit-ups and push-ups.
Good luck. Grade and Middle School Coaching is very rewarding. I doubt I will ever coach a kid with world class talent - even close. But we may be launching some of these kids into a lifetime habit of physical fitness. Definitely a worthy endeavor.
Good luck and congrats!