The Chronicles of Narnia, British
TV.com; Wikipedia
There were seven books by C.S. Lewis, in this series. Four of them were made into sort of TV movies or miniseries or whatever. Of course, anyone buying new sets of the books will find them in chronological order of the events in the story, rather than the original order in which they were written. Originally, the first book (and therefore the first movie/miniseries) was The Lion, the Witch, and the Wardrobe (IMDb). In it, four children, Peter, Susan, Edmund, and Lucy, travel unexpectedly through a magical wardrobe and find themselves in a strange land called Narnia, where they have to help defeat an evil witch and eventually they become kings and queens, and live to adulthood before finally returning to the normal world, where they are children again because time passes far more quickly in Narnia than it does here. Also there was a talking lion named Aslan who is like a god. Actually the books are sort of analogies for Christian stories. Aslan is basically Narnia's version of Christ, and his father is "the Emperor Over the Sea," I believe....
Anyway, later there's another movie/miniseries, Prince Caspian and the Voyage of the Dawn Treader (IMDb), which combines the 2nd and 3rd books in the series. The kids go back to Narnia and find centuries have passed since their reigns. Now they have to help young Prince Caspian reclaim rulership from like this evil uncle or whatever who had usurped the throne. Then there's this voyage on this ship which um... goes various places and has various adventures.
The next installment is the fourth book, The Silver Chair (IMDb). I don't remember the story too well. There's this kid Eustace who was introduced in the last movie, I don't remember if it was the 2nd or 3rd book. He and his friend Jill are the main characters, cuz now the other kids are too old to return to Narnia. I guess. And there was this guy named Puddleglum who was played by Tom Baker, better known as the fourth and best Doctor from Doctor Who. And that was the end of the series, the other 3 books were never adapted, alas. But you really should just read all the books, anyway. They're really classics and quite good. And I should see all the series again sometime and write a proper review....