If they’ve got a nice sound and decent flexible-just keep doing what you are doing. In time it will probably come. Patience.
While the others are hanging out in upper register land, maybe they can play the same thing in a better key. It might also make them want to practice a little more to do what their friends are doing.
If they can play a c with good tone, they’ve already got a range that will cover most 2nd band literature (at least for 1st concert) so not sure I would worry too much at this point.
Keep working and be patient. They’ll get it.