I don't know of any material specifically designed for security staff.
I suppose from a language point of view they would need to know imperatives, question forms, giving directions. I would focus most of the lesson on oral English, do a lot of work on pronunciation and give them plenty of practice listening to different accents.
You can talk to your students; they'll be able to tell you the situations they encounter most frequently. You could also try to arrange a visit to their place of work and spend some time on the job with them - could be fun!