What is Butler's role and how is this a reversal of whom he would be expected to be as opposed to Artemis's role in the mind of a reader?

Butler is the adult, so one would expect that he would be the criminal and the young Artemis would be either his apprentice or his lackey. Butler is also a large man making him appear to be the one who would make the decisions in a story based on criminal activity. Artemis is only twelve years old and should be home playing, not making the kinds of decisions that include tricking sprites with alcohol.