I don’t think there is any one single workout that I could say would be the magic bullet. Improvements will come first from consistently putting in the mileage over the winter in the form of steady runs which would increase in distance as they became fitter. You want to be get them to a high level of endurance so that they can step on the track and do repeats of anywhere from 200 to 1000 meters at or just a bit faster than their goal pace. When they can get to the point where they can run 6-8 x 400 in 70 seconds (with about 90 seconds rest) or so then it’s a good bet that they are just about ready to run under 5:00.
It’s tough to get too much more specific without knowing what they’ve done to this point in workouts, mileage etc.