In the commercial, the members' heads were superimposed on the bodies of six-year-old children. The concept behind the commercial is that it's supposed to be like the NHK kiddy shows that feature adults singing and dancing with children. Here, AKB48 member Yuko Oshima is the "big sister" type character.
Online the reaction in Japan is ranging from, "This is creepy" to "Holy crap, this is creepy."
You can watch the commercial in the above TV clip. It starts about 26 seconds in; there's also a making-of.