When we first discovered kitten soccer, we decided the regulation ball is a Ping Pong ball. That may not actually be true. We've got a crumpled ball of colored foil that's trying to establish a competing standard.
Foil seems to be easier to pull out of corners and other places where Ping Pong balls get trapped. The big advantage, though, is that foil can be picked up and carried as well as batted. Very useful when trying to keep the ball away from one's brother.
Hmmm. If the ball can be carried as well as passed, does that transform the game into kitten rugby? Especially since the game often degenerates to a scrum, also known as Cat Fu?