How did the grand high witche plan to get rid of the boy? Why?

With the mice dealt with, The Grand High Witch returns to the recipe. The recipe requires a witch to boil the wrong end of a telescope until it gets soft (to make the child small), to fry the tails of 45 brown mice, to simmer the mice in frog-juice for an hour, and to set a 24-hour alarm clock for 9am and then roast it until crisp and tender. Then, one needs to blend all of the ingredients in a mixer, add the yolk of a gruntle's egg, the claw of a crabcruncher, the beak of a blabbersnitch, the snout of a grobblesquirt, and the tongue of a catspringer. After cooking all of these things together, the mixture will become a green liquid. One drop of this will turn children into a mouse the next day at 9am, though an overdose could turn the child into a mouse instantly, which The Grand High Witch warns against since it could lead to the witch being caught.