Identify children's SF story from early '70s

I think the story was a trilogy from the children’s scholastic reader. Here are the things I think I remember about it.

First book 3 children brothers/sister exploring hillside and discover a cave entrance. They follow some type of path or stairs that go deep underground and discover some city of ‘mole people’.

Other books, their father was some kind of scientist that was presumed dead? The children discover clues that their father was stuck in some alternate dimension?

There was also something about an "X"; it was some kind of symbolic clue.