This is the only article of its kind in existence. I found it when I was doing an article on litter a few months ago, and I started researching around it and found that unless the kitten is actively eating a lot of the clumping litter it really isn't harmful. Though common sense suggests it is prudent to use regular clay litter, because most kittens will sample litter for some reason, but once they get bigger, unless they are ill, they stop doing it.
One veterinarian suggested that in order to prove this theory that clumping litter kills, the study itself would be so intusive that no one dares to attempt it. Because face it, this would mean force feeding clumping litter to kittens and cats!
Here is the article I foundhttp://www.lisaviolet.com/cathouse/litter.html