There is a series I read when I was a kid in the late 90s. The first book was literally a clone of The Lion, The Witch and the Wardrobe, plot points and beat for beat--kids get taken to strange land with talking animals and have to save it from evil witch.
The other books in the series were much different, though. In the second(?) book, the kids had to travel to some distant island and come back with some artefact. I distinctly remember them getting trapped by some sort of sentient mud monster called the "Earth-Trogg" in a cave before escaping.
In another book, they had to travel to the North Pole to stop penguins from completing some sort of ritual (the back of the book had "the penguins have stolen the north pole" as a tagline... which I found funny since it sounded comedic but the book was quite serious.)
In the last book, it was similar to Narnia's the Last Battle--all the enemies of our heroes have united, and they're trapped in an ancient fort on some mountain until saved by the "god" figure who rebuilds the world.