You don't mention having a boarding contract. I'd start there, and just let them know that because of the huge responsibility and liability involved in caring for an animal as large as a horse, you've decided that you need to formalize the situation more. If you need to, fib a bit and say your home owners insurance found out you were boarding and demanded a contract or they'll drop you
You can then put in writing when their board is due, and if need be, add a clause that says every other month $XX is due for worming and farrier fees. And maybe once per year (pick a month) $XXX will be due for shots/coggins and a holding fee (if need be). Be sure to include what they are paying you for (X lbs of grain, Y flakes/lbs of hay, Z bales of bedding, stall cleaning, turn out... whatever it is that you provide). Good luck!