Trivia: Roald Dahl hated the ending of the movie so much that he would drive past movie theaters with a megaphone and tell people not to watch it.
Trivia: A lot of the witches who attend the Grand High Witches meeting are actually men dressed in women's clothing.
Trivia: Roald Dahl was happy at learning that Anjelica Huston was playing the Grand High Witch as she was his preferred choice for the role.
Trivia: In the ending for the book, the boy stays a mouse, as opposed to the movie, where he becomes human again. Roald Dahl hated the movie ending.
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.