I couldn't find one all-inclusive website (but how awesome would that be!!? I hope someone finds it for you!), these may work. Not sure how kid/teen friendly they all are, but might be worth checking out:
http://urbangeek.net/dictionary/
The Dictionary of Imaginary Places was originally published in 1980, with substantial updates in 1987 and 2000. There are over 1200 entries in the book, ranging from Middle-Earth to Narnia, Oz to Earthsea, Hogwarts to Neverwhere. Despite the wondrous variety available, I was disappointed to see so many locales left out, so much uncharted territory. So I have taken it upon myself to fill in the gaps, and invite you to do the same.
The Librarian's Internet Index has a few selected websites for "Fictitious Places in Literature":
http://lii.org/pub/subtopic/1794
http://isbndb.com/c/Imaginary_Places/
ISBNdb.com project is a database of books providing on-line and remote research tools for individuals, book stores, librarians, scientists, etc. Taking data from hundreds of libraries across the world ISBNdb is a unique tool you won't find anywhere else. Please check the FAQ for more details.
for that patron obsessed with all things Vermont :o)
A Gazetteer of Vermont Places: Real and Imagined: http://crs.uvm.edu/community_data/gazetteer.htm
A really kid-friendly site is http://www.storybookengland.com/index.php It's an interactive atlas of England that maps out books like the Alex Rider series, His Dark Materials trilogy, and even Winnie the Pooh! The music is a tad annoying but the site is fun :o)