That's not dumbing down; it's a difference in usage between one side of the Atlantic and the other. "Philosopher" doesn't have the same connotation in the US as it does in the UK, especially to children, the book's target audience.
I was also thinking of the above example, I do think "Sorcerer" has a more childish ring to it than "Philosopher". But then again, we are talking about children's books so it's still just marketing towards a target audience.