story identification - 90's book about children climbing a tree and visiting different worlds from there

In the 90's I read a book about two children (I think they were siblings, boy and girl) who climbed a tree. At the end they found a door to another world. After some time the (fantasy) worlds switched to another one. Once they got stuck in a world, because the world went on traveling and therefore the door couldn't be reached anymore. They met many curious individuals during their adventures.

It's a completed novel for kids. I've read it in German, but I can't remember if the author was a German one, so it might've been available in English as well. It was a fairly thick book.

EDIT: After some thought I remember a pancake kind of person

Answer

I'm pretty sure you're thinking of The Faraway Tree, a series by English author Enid Blyton. Books in the series include The Enchanted Wood and The Magic Faraway Tree.

The "pancake person" was probably Moon-Face, named for his round face that looks like the moon.

In the first novel in the series, Jo, Fanny and Bessie move to live near a large wood. One day, they go for a walk in the wood and discover an enormous tree whose branches seem to reach into the clouds. This is the Faraway Tree.

When the children climb the Faraway Tree they discover it is inhabited by different magical people, including Moon-Face, Silky the fairy, The Saucepan Man, Dame Washalot, Mr. Watzisname and the Angry Pixie. They befriend some of these people, in particular Moon-face and Silky. At the very top of the tree they discover a ladder which leads them to a magical land. This land is different on each visit, because each place moves on from the top of the tree to make way for a new land. The children are free to come and go, but they must leave before the land moves on, or they will be stuck there until that same land returns to the Faraway Tree. In various chapters, one of the children gets stuck in the land.