Make sure the kids are not playing towards their midriff. There should be no obstruction of the instrument sound. Have them sit at a 45 degree angle to the chair. this helps with shorter players. They can move their right foot forwards or backwards to adjust the level and angle of the mouthpiece. Speaking of mouthpieces, try a Holton or conn MDC mouthpiece as opposed to a MC. A conn 2 is a good beginner mouthpiece. Make sure their right hand is in the correct position. I can't tell you how many horn sections I've stood behind and the hands were everywhere. One trick you can do: place a music stand behind the bell to project the sound. Get some music that features the horns with FF playing or a few rips, which horn players live for. Play something like the CATHEDRAL CHORUS from Reed's Slavonic Folk Suite. The horns make this movement out of this world. It is a simple 3rd space C down to an F, but when my section played it in HS and I was 1st, we sounded like a freight town was coming through.
If you have advanced horns, play something like Elsa's or Danza Final from Estancia, not to mention American Overture (and let them rip from G to the high G. What's the fun of a clean slur?)