Have them add a down beat every 2 or 4 measures. If they have:
r&r& r&r& r&r& r&r&

Add a down beat so it reads:
1&r& r&r& 1&r& r&r&

Or as they get better take out the 2nd measure down beat and make it once ever 4 measures.

Lets be honest, no judge is going to be listening to the horns adding in a downbeat note when you have lows playing all of the down beats at the same time (or they won't hurt your score because of it), but they will NOT like poor time from the horns because they can't keep good time.

Good luck!