If possible, can they be a part of that? Make sure it's something engaging and not just 8 on a hand on pads or the oboe part on xylophone. You can even write out little accessory patterns for them to play to accompany the band warm-ups.
2. Can you split them out with another director and do some keyboard or snare fundamentals? What about a percussion ensemble?
Even if you just teach them some additional lessons from their beginner book, they'll be more engaged by working on that than a lot of other things you could be doing in full rehearsal.
If percussion ensemble is an option, this can be a great way to work on multiple skills from the obvious chamber music applications to less obvious like learning about accessory percussion and trap tables.
Holler if you want to talk further: ericrathmusic at gmail dot com