Sure, the McAllisters have a large house, and its awfully convenient that they happen to have a whole plethora of items that come in perfectly handy when looking to thwart a couple of pesky burglars, but mannequins? With Kevin trying to make it look like theres more than just him at home, he sets up a faux party with music and a whole bunch of mannequins. Its not going out on too much of a limb to say that most regular households dont have even one mannequin, so where exactly does Kevin pull three of them from? And then theres the question of what happens to all of these after that scene. Does he put them away, and if so then why?