I have been both a coach, and a Scoutmaster for many years, so I've had more than my share of shepherding.....
First thing I would do, is use the "buddy system".
Partner each kid up with one other, and tell them they are to keep together
ALWAYS, and look out for each other.
How old are they? If they are old enough for cell phones, make sure you have each kid's number, and make sure that each "buddy team" has each other's number.
Ideally, you should have 1 adult for every 6-8 kids.
Assign each adult their share of "buddy teams" to watch, in addition to their
overall adult responsibilities.
Make sure all adults have each others cell phone numbers.
Instruct ANYONE in the group that, if they find themselves separated from the rest, they are to STAY PUT. Do not leave with ANYONE other than a police officer. If they are too young for cell phones, and see a police officer, they are to tell him/her they are lost.
Each kid should be issued a list of cell phone numbers to reach any adult in the group. Tell them you will re-collect the list when you return home, so they MUST take care of the list.
You should have health insurance info, and treatment permission documents
from a parent of each kid, as well as a contact/cell # for at least one parent/kid
If you are going in the wilderness, it gets much more complicated.....
EDIT: Not sure I addressed your question about something "big" happening, but it would depend on what happened