Huck knows the duke and king are frauds from the beginning. Why does he pretend that he thinks they are the real thing?

Huck knows that if he tells the con man that he knows they are frauds that it will cause trouble.  As long as they don't want anything ridiculous or push it too far, it is easier for Huck to just go along with their lie.  Huck is a practical person and he knows it will be easier to just go along with their plan.