Ha ! Momoka and Shiori are soooo cute ! And Reni-chan so... Reni-chan ! And that's maybe weird, but in the Shiori pic, it's Kanako who seems the most cute for me. :p
Their job is basically to appear a few years younger than they are. AFAIK Kawakami doesn't usually allow eyeliner, probably because it has this effect.
Not talking to the camera they sometimes sound like real people, too. Aarin's voice gets deeper and Kanako's grammar loosens up.