It looked like a child’s playroom: toys in cubbies, a little desk for doing homework, a whimsical painting of a tree on the wall. A woman and a girl entered and sat down in plump papasan chairs, facing a low table that was partly covered by a pink tarp. The wall opposite them was mirrored from floor to ceiling, and behind it, unseen in a darkened room, a half-dozen employees of the toy company Mattel sat watching through one-way glass. The girl, who looked about 7, wore a turquoise sweatshirt and had her dark hair pulled back in a ponytail. The woman, a Mattel child-testing specialist named Lindsey Lawson, had sleek dark hair and the singsong voice of a kindergarten teacher. Microphones hidden in the room transmitted what Lawson said next. ‘‘You are going to have a chance to play with a brand-new toy,’’ she told the girl, who leaned forward with her hands on her knees. Removing the pink tarp, Lawson revealed Hello Barbie.