Join Date: May 2008
Location: St. Albert, AB, Canada
How old are the kittens? If they're quite young it may be that they either don't know any better (are too young), or that the pan is too high for them to comfortably get into.
I would add a second litter pan, across the room from the first, and then place any stools left on the carpet into the litter pan to make it smell like the bathroom. if it's possible I'd remove any rugs as well, so the most welcoming spot for them to go will be the litter boxes.
Its also possible they don't like the litter you are using. Make sure it is non-clumping clay (or swheat scoop/worlds best), and unscented. Cats have very sensitive noses, and if I find some of the flowery scents too strong they must be completely overpowering to the cats.