Mrs. Whatsit asks for a Russian caviar sandwich andCharles Wallace angrily responds saying, “you peeked.” Why does the author choose these words to indicate Charles’ anger? What does this reveal about Mrs. Whatsit? Compare this revelation to Charles Wallace.

When Charles responds to Mrs. Whatsit's request for a Russian caviar sandwich with, "You peeked!" I think the tone is perhaps not quite angry, but closer to mock outrage. He has already, in this chapter, expressed his fondness for Mrs. Whatsit, referring to her and her friends as "very enjoyable." Charles is also, in the story, quite a precocious, even-tempered boy, meaning that it would be out of character for his exclamation ("You peeked!") to be genuinely angry.
When he accuses Mrs. Whatsit of peeking, he perhaps means to imply that she must have somehow seen the Russian caviar that the children were saving for their mother's birthday. Later in the story, we find out that Mrs. Whatsit can actually read minds, so perhaps Charles (who has met Mrs. Whatsit before and perhaps knows of her abilities) is accusing her here, playfully, of peeking into their minds and discovering in that way that there was Russian caviar in the house.