If there are enough beds so that the guests don't have to use the urine-stained mattresses, I'm not sure why they are so upset. I think that refunding one night's rent is ample compensation for them.
Going forward, make sure that you have good quality, water-proof mattress pads for each mattress, with extras in each size that guests have access to in case someone has an accident and the guests need to change the bed linens during their stay. I've never tried it, but there are lots of suggestions online regarding how to clean urine stains in a mattress. I'd try one of these before buying two new mattresses.
I also have to add - I would be very unhappy to discover that my guests are schlepping queen-sized mattresses from one floor to another in my vacation rental!
Urine stains = ICK. And what a shame that your housekeepers "missed or ignored.' (Not that this helps you now? But, do you have mattress protectors in place? Not to mention: a check-list for those housekeepers?)
Regardless -- and only because my circa 1860s home recently suffered -- your post resonated on account of Guests who scuffed = scratched the heck out of hardwood floors due to "furniture relocation." Never had I considered including a clause in my RA that prevents + penalizes for such re-decorating. It does now.
You say that "there are more than enough beds for the[se] guests excluding the 2 damaged mattresses." And kudos to you for offering to "relocate them to a resort, [and] with apologies + [a promise] to make it right," no less.
Curious ~ on the basis of 'what' have you made your good will gesture?
You can no more control pee pee accidents than you can control the weather. As others have mentioned, have water proof covers. Of course I will tell you, unless you buy those annoying vinyl covers there are actually no such thing as waterPROOF covers, been there done that. If you get water resistant covers they only work if the guests change the sheets rather quickly (and I can also tell you that many kids that have bed wetting issues either don't realize they did, or don't share because they are embarrassed.) Been there, done that.
At this point you either have to a)replace the mattresses entirely, or b) purchase some sort of cleaner that takes urine stains and smells out, knowing you will only be able to lift part of the stain/smell, as it has now penetrated the mattress completely. (unless you want to hire a professional to come out and steam clean and even then I don't know if you can actually get the entire mess out.)
Once you clean with this product, then the mattresses will need to be properly air dried, THEN cover them with water proof/water resistant mattress covers that your guest knows are not to be taken off. Get the kind that are bed bug proof as well. Then put a mattress pad on.
Again, been there, done that. Good luck to you!