The time for planning this was months ago, before kids and parents have paid for the trip.

Unless you're planning for next year, of course.

Your best bet is to consult with campus admin and district level administration at this point, because you could be on the hook for a ton of financial liability if parents refuse to abide by a "brand new code of conduct" thrown at them at the "last minute" by you.

I strongly recommend you put travel policy in your band handbook going forward, and include verbiage about overnight trips. It happens infrequently but it may happen even for football trips.