Open Question: How to bill for being on 24 hour call?

I am considering taking a side gig where I would be responsible for managing a group of college students coming to Beijing, where I reside, to do internships. Part of the arrangement would be being on 24 hour call in case of emergencies. I can only imagine the types of trouble study abroad university students can get into. I can give a fixed fee for all the other responsibilities, where the time commitment and the responsibility is clear. I want to have an idea on how to put a price tag on responsibilities that may or may not happen, and where time commitment and content is unknown if an emergency were to happen.