Peter and Susan won't return because they are too old to come back to Narnia. The journey they made in Prince Caspian was their last; Lucy and Edmund return with their cousin in The Dawn Treader. That was their last journey. After that, none of the Pevensies came back to Narnia until The Last Battle, which was the very last book in the series. In the last few books, the main characters are the Pevensies' cousin Eustace and his friend from school Jill.