On iOS, if you leave the hotspot open, no one will see any files on your phone. So no worries there. If you do use a password, I recommend a random string of at least 12 characters (20 would be better) that includes upper/lower case, numbers, spaces, and/or special characters.
For the record, I will most definitely be doing this during our next assembly. However, I have to stealth forget my hotspot on my wife's and other's phones. They already have access to my hotspot.