Have the kids run 400m as hard as they can. Those at the front or who go out very crazy fast are sprinters. Those at the back are distance runners. Make it fun and simple:
Sprinters: Sprint, rest, repeat. Coach on running form and using blocks. Some sprinters this young will actually be slower
Girls, boys, doesn't matter at this point. Girls will actually be more mature at this point. For distance runners focus on the actual time spent, not the distance. So a guy running 8:00 pace will cover 4 miles in 32 minutes while a girl running 10:00 pace will only cover 3 miles. Time...not distance.
Make the workouts a game, something they can compete in:
Paired 400s: pair a male and a female. Run a 400, jog around a circle until teammate gets back to you. Repeat for a mile each.
3-pair 200s: 3 person relay. 200m, jog around until partner gets around the track. Each person completes 8 200s.
5-pair 100s: 5 person relay. Good for all-out sprints. 100m, jog/walk around until partner catches up.
3-pair shuttle relays. Good sprint workout. 100m back and forth. 10 reps each.
For distance road work, try an Indian Run. Runners go single file. Every so often you signal for runner at the back to take lead, that runner controls pace no matter how fast or slow. Mix it up so runners go various speeds or distances. You can even mark each runner with a purpose (sprint, harder run, slow, etc)
Alan