I can recommend a few things:
1. Manna Pro Sweet PDZ Horse Stall Refresher Granular
. Be sure to get the granules of this and not the powder.
2. Bye Bye Odor Concentrate
I prefer this one, but ...
Okay, so the Manno Pro granules are easier to apply. You'll have to order this giant 40lb bag and lug it to your barn. However, you then sprinkle it around the stall. (I use an old Chinese food, quart-size, plastic soup container as the sprinkling device.)
I prefer Bye Bye Odor. For this you put a few capfuls into a spray bottle and add water (there are instructions that tell you the amount). Then, you mist the stall (preferably on the rubber mats before adding the additional bedding. So in your case, you may have to push the bedding aside, mist, push the bedding back and do this around the entire stall). My hand gets exhausted with that much spraying. Therefore, I eventually invested in a larger-size pump sprayer. Solo 418 One-Hand Pressure Sprayer, 1-Liter
Using this you can mix up a larger volume. Then you pump to load and can spray by just opening the nozzle. This is all much easier (overall), but you'll have to do this every day (or as frequently as you possible can).
So I agree with other posters. Although it is awful that your daughter smells like urine, the worst part of this is that it is doing very serious harm to your horses lungs.
Because of all of this, I'm a big proponent of horses living outside. However, I totally understand that that is not always an option. (But if it is an option for you, find good field board (and it's cheaper!)). Also, I understand that it is very tricky dealing with boarding facilities. (And they do not have an easy job dealing with horse owners, so I encourage kindness and fairness on both sides.)
If you need to pull the mats to clean/spray/spread granules under them, then I recommend these: EZGrip Mat Mover
So the granules are easier (in general). But the bag is heavy, and they are expensive! The liquid works better (from my experience), but you have to spray very thoroughly and regularly.
You may want to get a bag of granules, pull the mats, and sprinkle granules underneath of them. Then, follow up as frequently as possible (daily if you can) with the spray using the pump sprayer.
I hope this helps. If you need any more info, please let me know.