Ballpitts are never washed. Not even once. However, some kids pee in there. This means there is a 100% chance that all balls in ballpits have been peed on, and not washed at some point.
There are specialized machines for this that resemble industrial vacuum cleaners but even bigger. Cleaned balls are bagged and get poured back once the floor and walls are sanitized. Do the operators keep track of when this was last done? Yes, and they aren't proud of it so you won't see cleaning logs hanging nearby like at most mall toilets.
Speaking as a parent with a horrible experience involving rotavirus:
NEVER, under ANY circumstances should you allow your kid into a ball pit. Just fucking don’t, they are gross and your whole family will puke and shit for days.
Let me guess, youre not vaccinated and nor are your kids?
Rotaviruses cannot be killed by normal sanitizing and nearly all children will have it (and developed resistance) by the age of 5. It almost never affects adults.
Might as well not let your kids do anything at that point, ballpits are not better at spreading rotaviruses than anything else. Ballpits are not clean, but the toys, floors and walls at the daycare aren't either, not as far as rota is concerned at least
We all have every vaccine you can get. It’s possible I’m misremembering exactly what disease it was, but I promise you that a single instance of our kid in a ball pit ruined a vacation for two families.
Ballpits encourage contact with eyes, mouth, and nose, then spread it all around over the balls. They are especially difficult to clean. It would be difficult to design a better disease transmission vector if you were trying.
Rotavirus immunity is partial. You only need to come in contact with a strain too different from whatever your vaccine or childhood exposure was and you'll catch it again.
I got the vaccine as a child, and yet I caught it in my early 30s for the first time after moving to another country. Have you seen The Exorcist? I didn't know projectile vomit was a thing for real until then. I also didn't know about projectile diarrhoea. I wish I was still ignorant of these things
You'd be better off claiming that there's a 100% chance that at least 1 ball has urine on it assuming that 100% of all ball pits have at least one urine incident.