I had a junior high group years ago that had no trombones at all, just three solid baritone players. I actually divided up the trombone and baritone parts for sight-reading because I could with those kids. That year I had to make sure to avoid programming anything that had a trombone gliss, but on the whole it was one of my strongest low brass sections ever. Not a word was said on stage or in sight-reading.

Given the year that it is - I would just have them play the baritone parts. For all the judges know you do have trombones, and all of them were quarantined for contest. Next year you might need to be a bit more concerned, but you have nothing to worry about this year as long as your kids play the notes on the page well.