You can find the answer to this on the very first page of Chapter 23 which is entitled "Square Candies that Look Round." What the Oompa-Loompas are doing is painting faces on the candies. They are actually square, but Wonka is making a play on the British phrase "look round" which would be "look around" in American English.
Veruca Salt and her mother argue with Willy Wonka, because they do not understand the play on words. He insults Mrs. Salt and she starts yelling at him. That is when he tells her to shut up.