They are using you as a free babysitting service and treating you terribly on top of it.
The flip side is that if you view it as an opportunity for this child to get to be with someone who truly loves horses, knows how to handle them and can put her to work (filling water buckets, carrying flakes of hay, picking a stall) then it may be a huge boost to her psyche now and in the future. Because her parents are parasites and I have my sneaking suspicion that they aren’t very good parents. Not when the chips are down. But I would check out my homeowner’s and umbrella insurance (if you have horses at your home, you should definitely look into umbrella insurance anyway) and have the parents sign all sorts of things that say they are aware that horses are big, strong and can be unpredictable. A hold harmless if they want her under your feet at the barn.
I wouldn’t charge them any money because then it’s a business and the headache and costs go into the stratosphere.
You have the bill of sale, the horse is yours. DEFINITELY say no more frequently, or perhaps entirely. You have the right to do so.