Pre-season is good to work on speed endurance. I was a 300m hurdler in high school (we didn't have 400m hurdles in New England), and we often did what our coach called "16/16's" or "1600 meters in 16 minutes" (like repeats but can vary the distances). So the kids would run a 400m every 4 min, or a 200m every 2 min, or some variation. Example:
time = 0 min: start first 400m
time = 4 min: start 2nd 400m
time = 8 min: start 3rd 400m
time = 12 min: start 4th 400m
The time between the kid finishing the 400m and the beginning of the next interval is their rest. So they would get less rest as the workout progresses. For guys, I'd say you'd try to get them to hit 60s for each 400m and have them try to keep that pace throughout the workout. You can easily add more intervals, so it could be 20/20 or 24/24.
A ladder version can be done like the following:
time = 0 min: start 200m
time = 2 min: start 400m
time = 6 min: start 600m
time = 10 min: start 400m
time = 14 min: start 200m
All done at 85 to 90%
You could also add in some 800m too, if you think it will help them out.
I also recommend that during the indoor season, have these 400m hurdlers work more on flat races with some basic hurdle drills 2x a week (to keep their hips flexible and moving, while improving on their form). Some basic hurdle drills would be walk-overs (10 hurdles in a row), lead leg and trail leg drills against the wall with a single hurdles, and then lead leg or trail leg skips alongside the hurdles.
I don't know what indoor events you have (or if you have an indoor team), but get them to race anything between 200m and 600m in a meet, including 4x400m relays.
Hope this helps. Good luck!