Dear K.,
I've chaperoned about a million school field trips. It's really a lot of fun!
Show up excited to get on with it!
The teacher will tell you what you need to know. Usually, they give you a group of children and you just keep track of them, keep count of them and go with the flow.
I usually got the unruly kids because I wasn't timid about keeping them from running off or doing things they weren't supposed to be doing.
If they take lunches, you just want to make sure everyone has their lunchboxes, nobody forgets their jacket. Stuff like that. You might have little kids that want to hold your hand or sit by you. (My daughter didn't really like that part at first, but she got over it).
Just have a great time! I'm sure your son will be very proud to have you there!
Best wishes!