[9991]

I don't know for sure, but I don't think so. The balls are very light, so there's almost no buoyancy. The only way to support yourself is if the balls are locked in place from the force of your foot stepping on it, which might work for a stack 2~3 balls in height at the most before it becomes unstable and slips out from underneath you.

[xwdv]

So presumably there could be a very deep ball pit with smooth walls somewhere in the world that is filled with the rotting skeletons of dead children at the very bottom?

[9991]

With enough kids they’d stack corpses to climb out.

[xwdv]

Indeed, this would make a good high concept movie, taking place entirely within a ball pit, kind of like that movie Ryan Reynolds did buried in a box underground. Good jump scare when a kid finds a dead face while rummaging through the balls.