@Cheap-Sleep0 During the pandemic - which, incidentally, has not yet gone away - it was a requirement (here in the UK, anyway) that mattresses, duvet covers, and pillows should not only have protectors on each item, but that they should re changed and laundered at every changeover.
Whilst many might feel that that was a step too far, we had been doing all that before Covid-19 struck, and to be perfectly honest, it's like many other things, in the sense that to begin doing something not done before seems rather onerous, once one gets into the rhythm of regularly doing it, it's just part of the job.
OK, so what happened in your case was nothing to do with Covid-19, but the principle is basically the same, inasmuch as each person who sleeps in a bed doesn't really want to believe that between that freshly laundered pillowcase, or the duvet that's on top of them, or the mattress that they're lying on, has not long had someone else (maybe sweaty, maybe with impetigo, maybe who.... well, hopefully you get the picture) sleeping there!
I mean to say, sheets, duvet covers, pillows are laundered after each Guest has left, but everything I mentioned does - to s certain extent - filter through those products, and whilst the protectors do do what they're in place to do (so the pillows, mattresses, and duvets are protected, I wonder how many Hosts ever give a second thought to the protectors themselves?!
And then of course, there's the wees - and it's not just the age-groups you mentioned that might have a leakage, however small, but there's always a chance that babies might have the nappies/diapers changed whilst lying on a bed, or there's younger aged individuals who sometimes have problems with pelvic muscles - but none of your Guests' would usually disclose such information to any Host, after all, quite often even their partners' aren't aware of such, as it's not something people in general want to talk about!
So the best answer - and the "cheapest" one as well - is for a waterproof protector to be put on each of your mattresses.
What you will also have to consider however, is that not only did the child exude a lot of wee, and that it soaked the mattress, but with the child moving about on the bed (or even if it didn't) the duvet will also have been affected by the urine, whilst if the child's hands became wet, then it's more than likely your pillows were also affected!
Yes, I appreciate that cost does come into the equation, but so does the wellbeing of all of your Guests, and to my way of thinking, it's far "cheaper" in monetary terms to purchase and put on the correct type of protectors to do just that; protect the components of a bed; protect your Guests from anything "nasty" or "not-very-nice"; and/or protect you from any bad comments/reviews or, worse; - than not.
I noted the price you mentioned per protector, and o appreciate you are in Iceland, but is there not one of those worldwide Companies with the name of a famous river in South America, in your Country, or are you not able to access one, or is it that even they charge that much for waterproof protectors?!
Hope some at least of the above will be of help.