What age would you start reading Harry Potter to kids? I'm thinking DS is still a bit young. He's 4 but still quite immature compared to other 4yo's I've met. Maybe Enid Blyton would be better. I think he'd like the far away tree books.
The concepts are quite mature. I worked in a primary school book supplier and the first three books were put into schools for the upper primary levels. The remaining books were then on a school by school basis - we would share the themes with them and then they would decide. The final book was shunned by most that we talked to due to what happens to Hermoine. I have read them all and have no plans to read them to DD1 for a few years yet (she's also 4) - not because I don't think she wouldn't grasp some of the story, but because I want her to fully enjoy it for everything that it is.