Has to be the one in Mickey's Toontown Fair, the one that's called Pete's Garage? I'm not sure you can pin the blame on all the kids, since the one near It's a Small World in Fantasyland is much cleaner (right next to Ariel), but there has to be some plausable explanation for the Walmart-like conditions of this bathroom. You need hip waders in the place by midday.
Pssst. Want to find a clean bathroom in any of the themeparks? Try the ones in the baby care centers. If any of your kids are under five-years old or so, no one will notice. Restaurant bathrooms, even those that are counter-service only, are generally cleaner as well, but keep in mind that most table service places won't just let you walk in to use the bathrooms.
Just how my mind works on a day when I didn't get any sleep the night before due to little kids waking up.