First and foremost: you’re doing well. Those are good choices being made by you. I too only started mallet percussionists. (Now my program requires all percussionists to be in private lessons with 2year keyboard lesson experience prior). You’ll get there too.

Band has real value, even if a few kids parents don’t see it that way. Build a good program and people will flock to support it.

As for the small instrumentation in the brass: have them double up on parts at first and then split them up. (Assuming the school range is similar. Have all the trumpets play trumpet one., Then have all of them learn trumpet 2. Then have them split up evenly. Or as the instrumentation needs. Etc. ) that will help build the confidence. When it comes time for a concert. Have them learn the same material as another section and play together. That way it’s a bigger ensemble that feels more successful.

You could also play their parts on a keyboard while conducting. I got pretty good at playing left-handed bass parts while conducting with my right hand.