Personally, I see nothing wrong with starting younger readers off with whatever interests them, or whatever YOU can make sound the most interesting to them. When I was in the first grade, I vividly remember my father describing the story of the Hobbit to me as a bed-time story one night. I asked him if we could read it, and he went out like the next day, got a copy, and read a few chapters out loud to me (with me reading over his shoulder) every night before bed until the book was done.

I then went on to read it and the lord of the rings on my own when I was in the second grade. Shocked the hell out of most of my teachers at the time because it's generally considered a much higher level read. This got me hooked on fantasy novels (and reading in general) ever since, and I believe the incentive this provided to learn/read/study at a much higher level than age appropriate material THROUGHOUT my scholastic career is what made me so successful in school.

So while Chronicles of Prydain is a good younger series that I haven't thought about or read in years. There is no reason you can't start a VERY young child with a much more "adult" level read. Children are far more capable than we sometimes believe.

On a more general level, if you were trying to stick to 'age appropriate' material, I'd say for younger readers those would be a perfect start. I'm also going to have to agree with Shanarra and Xanth also, I absolutely DEVOURED Xanth books in middle school because I found them hilarious.