While I am very familiar with Shirley Jackson's work, when reading "Charles" I was completely thrown off guard. While I certainly should have expected something to be brewing under the surface, I was completely at ease when reading.
Curiously, while your question was posed a while ago, I found it very intriguing. Why was I so relaxed when reading the story?
After some thought, I decided that the story simply seemed too familiar to me. I have heard about children acting up when first beginning school. I found no threat that the story would end up being so twisted and surprising. The fact that Laurie, the boy with a girl's name, created a very masculine alter ego simply made sense after reading the story. I simply did not read into it until after finishing.
Therefore, while very familiar with Jackson's texts, the story (unlike other works of hers) put me at ease. Too much at ease for the end. Therefore, it was not really my knowledge which helped me enjoy the story. Jackson simply put me too at ease for me to suspect anything. In reality, I should have known better.