I very much disagree with this. The vast majority of people on here who were unhappy with Prince Caspian as a movie were upset that it deviated grossly from the original spirit and meaning CS Lewis had when he wrote the book, NOT because they were expecting something darker. EVERYONE (from directory Michael Apted, to the actors, to the promotors) agree that the strategy should be to return to the simple family friendly magic and beauty of the LWW, rather than continuing to go down the route of making it LOTR-lite like PC was. Everyone associated with the film thinks that it did poorly because they tried to compete with movies like Iron Man and Indiana Jones, and they realized that what they were doing was trying to stick a square peg in a round hole. Even the release date (Christmas for families, rather than summer appealing more for teens) reflects this.
Furthermore, I disagree with your proposed strategy of trying to appeal to the one growing age group. The books are primarily children's stories, but they appeal to all ages; if they try to make them more "adult" and trample on the original spirit of the how the books were written, the backlash will be severe, (as it was with Prince Caspian). Plus, why shouldn't they appeal to a whole new set of young fans each time for each new film? People who already liked the first couple are not going to easily lose interest in them later on-especially if they remain true to the books, since the books continue to be loved by people of all ages. The CoN transcend any one age group, and I'm glad to see that they seem to be keeping this in mind with VDT, and I hope it continues this way.