Continuity mistake: The position of the hood on the baby stroller as it rolls down the cliffside hills keeps changing from fully down, to halfway up, etc. as Luke chases it.
Continuity mistake: The position of the drawing Luke holds, as he lies down on the bed talking to his Grandma, changes. Also, the Grandma (Helga) picks one drawing up twice between shots.
Continuity mistake: A small brown card appears in the witch's hand, as she stands up on the stage and says that she smells dogs droppings, as the other witches are leaving.
Continuity mistake: After a witch is blown up in the RSPCC meeting, the size of the burnt hole in the chair changes for a shot, and so does the pile of black charred remains under the chair too.
Continuity mistake: The way that Helga holds her bag as she waves it in front of the Jenkins' parents changes between shots, as she tells them she has Bruno inside it.
Continuity mistake: The amount (level) of chips in the bowl, which Luke (as a mouse) hides in keep changing, as the chef puts the bowl down on the trolley, to when the chef picks it up and carries it away.
Continuity mistake: At the end of the movie when the Grand High Witch's assistant watches Luke and his Grandma drive off from the window, she pulls the curtain and is not wearing gloves.
Continuity mistake: When the evil witches start turning into mice, there is a close-up shot of the Woman in Black with mouse ears and whiskers, and her wig has fallen off. When the Grand High Witch is holding the spoon, there is a background shot of her dining room table, and the Woman in Black's wig is back on her head.
Continuity mistake: When Grandma says "Bruno's dad is about to have the soup", her right hand appears/disappears holding the bag depending on the shot.
Continuity mistake: After Grandma swings her closed bag in front of the Jenkins, angle cuts to a close-up of the bag and it's open.
Continuity mistake: Bruno's dad is about to have the soup and says "this is a nightmare", his wife is either holding the glass of wine or not, depending on the shot.
Continuity mistake: When Grandma says "Bruno's dad is about to have the soup" she is either holding the bag above the table, or the bag is resting on the table and her hand is gone.
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.