This is a series of at least 4 books, which I read in about 1965. A family of 3 children found a secret world they could enter under or by a river. I can’t remember much about the secret world, except there was a nice woman they visited. Not much to go on!
Stump the Bookseller
New format, same book stumper solving.
Is the book called The Secret World of Og?
It sounds a bit like New Zealander Maurice Gee’s trilogy, Half Men Of O – hes one of my favourite writers. Hope this helps!