Oh, that is a horrible place for a diaper to leak (plane seat being another one, especially for long trips).
If the parents had water with them, or maybe a little cloth, they could have taken the action of wiping it down or putting water over it and cleaning it off. But generally, people disregard their own selves when being a part of public property, even a candy wrapper is thrown outside of trash cans which are usually located everywhere.
You saw what happened and instructed your kids to move from the slide. What if you hadn't seen it and no action was taken from the other side. Also, parents need to be extra careful of ensuring that their child is safe from germs and diseases that can be spread with an incident that might be insignificant in their eyes but when in public areas, they are all that matters.