I think the appeal of the books is the simplicity (even crudity) of the writing and the story lines. They have no moral ambiguity. Good people are very good and rotters deserve punishment. It inevitably comes their way. It appeals to a kid's sense of justice and security. It is escapism, unencumbered by any real sense of likelihood or danger.
I liked them a lot as a child and now my son revels in them and gets really excited when the rotters are about to get jugged. Yes, the writing is garbage. I am glad Jonah is getting so much enjoyment from them now. He won't always be six years old.