Apparently, kittens are able to service any metaphor in Jossverse. Back in season 3 a kitten represented Buffy'n'Faith's connection. In season 5 miss Kitty Fantastico was a symbol of Willow/Tara mystical bond. In s6 they serve as a foreshadowing of Potentials. Right now they're artificial, vampy and evil and may represent toy industry, human's inner vampire or even sperm as curiouswolf36 suggests upthread... :)