OP, The trainer gets to use your horses for free? That is mighty generous of you. Consider having her pay part of the “use” fee for your horses at a show (maybe taking the money out of her day fees).
I can see where students might want to go to local shows if they are taking regular lessons. Will the trainer haul the horses (charge clients) and collect day fees? Will you haul and collect hauling fees? Does the trainer really want the students to show or is she just willing to coach them at shows and collect day fees if they actually go? Are your horses schoolmasters or school horses, and are we talking training/first level? How well is the whole deal working out for you?
I’m posing all of these questions because I think the answers would factor into how I would proceed. If the trainer’s business would benefit from her taking these students “out and about”, maybe she’s willing to participate in the “leasing” of your horses for a day. She’s getting a sweet deal by using your horses for lessons for free, and she might come out ahead at the end of a show day. Also, there will be a limit at which the average person who only takes lessons and wants to occasionally show will pay to actually go to that show. At ~$30-45/class, office fees, hauling fees, trainer coaching fees, horse leasing fees… will add up to a very expensive schooling show for the students. Maybe they don’t care!!
I think it’s fair to charge for your horses to leave the property and go to a show, but I think the price would depend on so many factors.