Other mistake: When the evil witches get their just desserts, Lois Leffour's friend Julia Whitman is choking on the Formula 86 bottle. Even though the bottle is small, Julia should have either seen it on the spoon, or felt it when she put it in her mouth. Instead, Julia swallows the bottle and coughs it up during her transformation into a mouse.
Other mistake: Before the Grand High Witch starts the meeting, Luke is watching the witches sit down, and he doesn't recognize the Woman in Black. This doesn't make sense because the Woman in Black attempted to kill him during the treehouse scene before any of them arrived at the hotel. The Woman in Black is even the first witch who tries to capture him when he runs on the stage.
Other mistake: When the Grand High Witch, the Woman in Black, and the other witches are counting down the seconds for Bruno's transformation, Luke temporarily steps out from behind the screen. Yet none of the witches in the front row notice him.
Answer: She probably didn't know about her master plan, just about turning kids into mice. Most likely she thought they'd be kept in locked cages and held prisoners.
During the meeting, Miss Ernst says that every child in England must be "Rubbed out. Destroyed." A clear indication that she was talking about killing the children after they get turned into mice but, Miss Irvine doesn't seem shocked or upset after hearing this.