If your kitties are like mine they just like to lay around the house, have a late brunch, lay around some more. Therefore, little shortie cat pajamas would be perfectly adorable!
Or, for those cats (or owners) with a more historical bent, perhaps longjohns for the boys and Victorian nighties for the girls. Both with coordinating night caps.
I don't know how do-able this is but I'm throwing it out there anyway!