I think that Rowling depicts a very fluid relationship between didacticism and child agency in the first novel of the series. I don't think that Rowling is driven by a need to present didacticism as an integral part of the novel. The story revolves around the fantasy elements of Harry and Hogwarts, and the eventual discovery of his own identity. There are elements in this narrative which can represent a sense of the didactic. Harry understands that his power must be used for good and he must challenge the forces of evil. Dumbledore is seen as the "epitome of goodness," while there are specific rules that govern Harry's life (when to use his powers, under which conditions can he be who he is.) In this regard, there are some elements of didacticism that even the talented Harry Potter must follow. Rowling does recognize the need to provide some structure and sense of order in a world where child agency and the ability to challenge authority is presented.
While there are elements of didacticism in this presentation, I don't think that Rowling lets these forces move in the way of child agency. It is evident that much of Rowling's focus in the novel is the presentation of how child agency can be transformative. Harry is initially presented as a child who "didn't know he was a wizard." The premise of the novel is for Harry to expose his own sense of power and his own sense of agency within it. The entire finding of the philosopher's stone is an exercise in child agency, as Harry, Ron, and Hermione challenge the normally didactic expectations of what children can do in their quest. Hermione has to break away from her didactic approach of "book smarts" in order to help Harry and Ron, and discover her own sense of confidence. Rowling depicts child agency as an essential component of identity formation. Didacticism is a part of this, but it is not shown as a force to impede such understanding and awareness. For Rowling, the path through which Harry better understands the world and his place in it is activated through child agency. Didacticism is a part of this structure, but it is secondary to the journey to better understanding the cosmic structure of the world and self.