I would suggest for them to do running on surfaces such as sand, hills, grass and stairs. As for a circuit training, at least a couple of days per week until their race day it would be a good idea if you have them run from station to station and perform their designated exercises. On the stations have them perform a certain number of repetitions (maybe 15-20) then run, not walking to the next station and perform the exercises in that stat station. Some of the exercises I would suggest are burpees, push-ups, jumps, crawling, carrying weights if are available (otherwise a backpack with some weights in it would be fine). You could also suggest for them if they have access to stairs to start incorporate them into their training regimen. I would have them do different exercises on each floor just like in circuit training.
I would suggest for them to do as much of their running training not on the treadmill. Even though running on treadmill is an option, it’s not enough to get prepared for an outdoor race like Warrior Dash, Tough Mudder or Spartan Race.